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Absolute Monarchyideology · politics · Early modernThe doctrine that legitimate political power should be concentrated, whole and unchecked, in a single hereditary monarch.
Accelerationconcept · physics · Cross-eraAcceleration is the rate of change of velocity, and because velocity has both a size and a direction, an object accelerates whenever it speeds up, slows down, or merely changes direction, even at constant speed.
Act Utilitarianismconcept · ethics · EnlightenmentThe right act is whichever one, in this particular situation, produces the most overall good.
Action-reaction pairsconcept · physics · Early modernEvery force one object exerts on a second is matched, at the same instant, by an equal and opposite force the second exerts back on the first, and this pairing is what makes pushing off anything, including empty exhaust gas, actually work.
Activation Energymodel · logic · Cross-eraNothing changes until it clears its starting barrier, so the leverage is in lowering the barrier, not in pushing harder.
The Acts and Omissions Distinctionconcept · ethics · Late 20th centuryThe contested claim that doing harm is morally worse than merely allowing the same harm to happen.
Adam Smiththinker · economics · EnlightenmentThe moral philosopher who gave the modern world its first systematic account of how markets work, and why they can serve the common good without anyone intending it.
The Adolescent Brainconcept · psychology · Late 20th centuryThe theory that teenage recklessness comes from timing: a reward and social system that matures early, running ahead of a self-control system that matures late.
Advaita Vedantatradition · religion · MedievalOnly Brahman is ultimately real; the world of difference is appearance, and your innermost self is not separate from the absolute but identical with it.
Adverse Selectionmodel · economics · Late 20th centuryWhen one side of a deal knows the quality on offer and the other cannot, the hidden bad drives out the good until the market itself unravels.
Alexis de Tocquevillethinker · politics · 19th centuryThe French aristocrat who saw, earlier and more clearly than anyone, both the promise and the peril of democratic equality.
Alfred Adlerthinker · psychology · Early 20th centuryThe psychiatrist who broke with Freud to argue that people are pulled forward by a striving to overcome felt inferiority, not pushed by buried sexual drives.
Alienationconcept · philosophy · 19th centuryThe condition in which people are separated from their own labor, its products, their human potential, and each other by the structure of economic life.
Amartya Senthinker · economics · Early 20th centuryThe economist and philosopher who reframed development as the expansion of human capabilities and freedoms, not the growth of national income.
Anarchismideology · politics · 19th centuryThe claim that the state is not a necessary evil but an unnecessary one: that human society can be ordered by voluntary cooperation rather than by centralized coercion.
Anarcho-Capitalismideology · politics · Late 20th centuryThe claim that no state is legitimate, and that every service the state provides, including law, courts, and defense, can be sold on a competitive market.
Anarcho-Communismideology · politics · 19th centuryAbolish the state and private property together, and let free communes distribute by need rather than by wage or price.
Anarcho-Syndicalismideology · politics · 19th centuryA revolutionary form of socialism in which workers seize and run the economy directly through their own federated unions, dispensing with the state altogether.
Angles, rotation, and radiansconcept · mathematics · Cross-eraA radian measures rotation as the arc length it sweeps out divided by the radius, a definition that makes the same amount of turning read identically on any size circle.
Angular momentumconcept · physics · Cross-eraAngular momentum is rotational inertia times angular velocity, and it stays constant whenever no outside torque acts, which is why spin speeds up as mass pulls inward.
Antifragilitymodel · logic · ContemporaryBeyond resilience lies a third state: things that do not merely survive disorder but grow stronger from it, and can be built on purpose.
Antonio Gramscithinker · politics · Early 20th centuryThe Marxist thinker who argued that ruling classes maintain power less through force than through cultural and intellectual leadership, a dominance he called hegemony.
Arguments for the Existence of Godconcept · philosophy · MedievalThe three great proofs of natural theology: that the world needs a first cause, that God is a being whose very concept entails existence, and that design implies a designer.
Aristocracyideology · politics · AncientRule by the best: government entrusted to a hereditary or qualified elite held to possess the virtue and judgment the many lack.
Aristotlethinker · philosophy · ClassicalThe philosopher who tried to organize all of knowledge, from logic and biology to ethics and politics, by starting from what can be observed rather than from a realm of perfect Forms.
Arithmetic operations as transformationsconcept · mathematics · Cross-eraEach arithmetic operation is a transformation performed on a quantity, addition and multiplication combine two quantities into one, subtraction and division reverse a combination to recover a missing part.
Asch's Conformity Experimentsexperiment · psychology · Late 20th centurySolomon Asch's line-judgment studies, showing that people will call an obviously wrong answer right to avoid standing alone against a unanimous group, and that a single dissenter breaks the spell.
Ashtavakra Gitatext · religion · AncientA dialogue in which the sage Ashtavakra tells King Janaka that he is already free, already pure consciousness, and that the only obstacle is the belief that there is an obstacle.
Attachment Theoryconcept · psychology · Late 20th centuryBowlby's theory that infants are wired to seek a caregiver's protection, and that the caregiver's responsiveness, first measured by Ainsworth's Strange Situation, sets a template for how the child expects relationships to work.
The Austrian Schoolideology · economics · 19th centuryA tradition holding that a market economy coordinates the dispersed, subjective knowledge of millions through prices, in a way no central planner could match.
Authoritarianismideology · politics · Cross-eraRule by concentrated power with little political pluralism and weak accountability, but short of the total control of private life that totalitarianism seeks.
Authorityconcept · politics · ClassicalAuthority is the recognized right to command and be obeyed, distinct from mere power because it rests on a claim to legitimacy.
The Balance of Powerconcept · politics · Early modernStates tend to align against whoever is getting too strong, because no one wants to live at the mercy of a single dominant power.
The Bargaining Model of Warmodel · politics · Late 20th centuryBecause war is costly, a settlement both sides prefer should almost always exist, so war is a failure of bargaining, not its natural outcome.
Base Ratesmodel · logic · Late 20th centuryThe prior frequency of a thing in the population, which the mind reliably ignores when handed a vivid particular.
Baumrind's Parenting Stylesconcept · psychology · Late 20th centuryA typology of how parents combine control and warmth: authoritative, authoritarian, and permissive, later joined by neglectful, each tracking a different profile of child outcomes.
Bayesian Updatingmodel · logic · EnlightenmentA rule for changing your mind by the right amount: start with a prior belief, see the evidence, and revise in proportion to how strongly that evidence points.
The BCG Matrixmodel · economics · Late 20th centuryA 2x2 that sorts a company's businesses by market growth and relative market share to decide which to feed, which to milk, and which to sell.
Beams, columns, and bucklingconcept · engineering · Cross-eraA long slender member loaded in compression can suddenly bow sideways and collapse at a load far below the load that would simply crush its material, and buckling is the study of when and why.
Bearings, bushings, and friction managementconcept · engineering · Cross-eraA bearing or bushing is the joint that lets one part rotate against another while a designer controls, rather than eliminates, the friction between them.
Behaviorismtradition · psychology · Late 20th centuryThe movement that tried to make psychology a science of observable behavior alone, ruling the mind out of its own subject matter until the mind forced its way back in.
The Big Five Personality Traitsmodel · psychology · Late 20th centuryFive broad, statistically independent dimensions, openness, conscientiousness, extraversion, agreeableness, and neuroticism, that keep reappearing whenever personality description is factor-analyzed.
The Biology of Stressconcept · psychology · Late 20th centuryThe body's response to threat is a two-speed cascade, a nervous surge and a hormonal one, that saves us from acute danger and wears us down when it never shuts off.
The Biopsychosocial Modelconcept · psychology · Late 20th centuryIllness and disorder arise from biology, psychology, and social circumstance interacting at every level, and diathesis-stress spells out the mechanism: a vulnerability that only becomes disorder under enough strain.
The Black Swanmodel · logic · ContemporaryA rare, unpredictable, high-consequence event that we insist, only afterward, we should have seen coming.
Blue Ocean Strategymodel · economics · ContemporaryStop fighting rivals in a crowded market; create uncontested market space where competition does not yet exist, by delivering differentiation and low cost at once.
Bottlenecks and the Theory of Constraintsmodel · economics · Late 20th centuryThe single slowest step sets the pace of the whole system, so the only improvement that matters is the one made at the constraint.
The Brain in a Vatexperiment · philosophy · Late 20th centuryHow could you tell, from the inside, that you are not a disembodied brain in a tank being fed a lifelike stream of fake experience?
The Buddhathinker · religion · AncientA teacher who diagnosed suffering as rooted in craving and a mistaken sense of self, and prescribed a disciplined path out of it.
Burnoutconcept · psychology · Late 20th centuryA syndrome of chronic, unmanaged workplace stress with three faces: exhaustion, cynicism, and a shrinking sense of what you accomplish.
The Bystander Effectexperiment · psychology · Late 20th centuryThe more people who witness someone in trouble, the less likely any one of them is to help, because each assumes someone else will.
Campbell's Lawmodel · logic · Late 20th centuryThe more heavily a quantitative measure is used to make high-stakes decisions, the more it corrupts and distorts the very thing it was meant to track.
Capitalismideology · economics · Early modernAn economic system in which capital is privately owned and deployed for profit, and markets, not planners, coordinate what gets produced.
Care Ethicsconcept · ethics · Late 20th centuryMorality is not first about impartial rules but about attending to, and taking responsibility for, the people we are actually bound to.
Carl Jungthinker · psychology · Early 20th centuryThe psychiatrist who broke with Freud to argue the unconscious is not only personal but collective, patterned by inherited, universal forms he called archetypes.
The Categorical Imperativeconcept · ethics · EnlightenmentThe supreme rule of morality: act only on a principle you could will everyone to follow, and treat every person as an end, never merely as a means.
Categories and Prototypesconcept · psychology · Late 20th centuryThe finding that everyday categories are organized around best examples and graded membership, not the strict defining features the classical view assumed.
Center of mass and balanceconcept · physics · Cross-eraThe center of mass is the single point where a distributed body's mass can be treated as concentrated for the purpose of predicting how the whole body moves and whether it stays balanced.
Change Blindness and Inattentional Blindnessexperiment · psychology · Late 20th centuryA person in a gorilla suit can walk through the middle of your visual field, in full view, and you will swear afterward that nothing was there.
Checks and Balancesconcept · politics · EnlightenmentEach branch of government holds specific powers to restrain the others, so that no single center of authority can dominate.
Chesterton's Fencemodel · logic · Early 20th centuryBefore you tear down a fence, find out why it was put up; the reformer who cannot explain a thing has not yet earned the right to remove it.
The Chinese Roomexperiment · philosophy · Late 20th centuryA man who processes Chinese by rulebook without understanding a word shows that running the right program is not the same as understanding.
The Circle of Competencemodel · logic · Late 20th centuryKnow the perimeter of what you truly understand, and operate strictly inside it, because the boundary matters far more than the size.
Circles, radius, and piconcept · mathematics · Cross-eraEvery circle's circumference divided by its diameter equals the same fixed number, pi, a ratio fixed by the geometry of flatness rather than by any circle's particular size.
Circular motion and inward accelerationconcept · physics · Early modernMoving at constant speed along a circle still requires acceleration, because velocity's direction is constantly changing, and that acceleration always points toward the center.
The Civic Cultureconcept · politics · Late 20th centuryStable democracy rests less on formal institutions than on a particular mix of citizen attitudes: engaged but deferential, participant but trusting.
Civic Republicanismideology · politics · AncientFreedom is not a possession you hold in private; it is an achievement that survives only where citizens actively govern themselves.
Class Struggleconcept · politics · 19th centuryThe claim that the conflict between those who own the means of production and those who work them is the central dynamic of history.
Classical Conditioningexperiment · psychology · 19th centuryPavlov's discovery that pairing a neutral signal with a reflex-triggering stimulus teaches the nervous system to treat the signal as a cause in its own right.
Classical Liberalismideology · politics · EnlightenmentThe doctrine that the individual is the basic moral unit, free by right, and that government exists only to secure that freedom under law, and no further.
Cleavage Theorymodel · politics · Late 20th centuryParty systems are frozen fossils of old social conflicts: the divisions that split a society when mass democracy arrived became the party families that outlived the conflicts themselves.
Cognitive Dissonanceconcept · psychology · Late 20th centuryThe discomfort of holding two cognitions that clash, and the pressure it creates to change one of them so they fit.
The Cognitive Model of Emotionconcept · psychology · Late 20th centuryIt is not events that upset us but our interpretation of them, and systematic distortions in that interpretation produce distress.
The Cognitive Revolutionconcept · psychology · Late 20th centuryThe mid-century shift, led by Chomsky, Miller, and Neisser, that put unobservable mental states back on psychology's table after fifty years in which behaviorism had ruled them out of bounds.
Collectivismideology · politics · 19th centuryThe family of views that treats the group, not the individual, as the primary unit of value, agency, or explanation.
Communismideology · politics · 19th centuryThe goal of a classless, stateless, moneyless society of common ownership, and the movements, humane and murderous alike, that pursued it.
Communitarianismideology · politics · Late 20th centuryThe self is not a free-floating chooser but is constituted by community, tradition, and shared goods, so a good society must cultivate them, not merely stay neutral among them.
Commutative, associative, and distributive structureconcept · mathematics · Cross-eraThe commutative, associative, and distributive laws state precisely which rearrangements of an arithmetic expression are guaranteed to leave its value unchanged, and part of what makes an operation interesting is which of these guarantees it fails.
Comparative Advantagemodel · economics · 19th centuryEven if one party is better at producing everything, both gain by specializing in what they give up least to make, and trading for the rest.
Compoundingmodel · economics · Cross-eraWhen returns are reinvested, growth builds on itself, producing an exponential curve that human intuition, tuned to straight lines, badly underestimates.
Confederalismideology · politics · Late 20th centuryA union of states that keep their sovereignty and lend the center only the powers they choose, retaining the right to take them back.
Confirmation Biasconcept · psychology · Late 20th centuryThe tendency to seek, read, and remember evidence that fits what we already believe, and to hold the rest to a higher standard.
Confuciusthinker · philosophy · AncientThe teacher who held that a good society is built not by law or force but by cultivated people fulfilling their roles with humaneness and ritual grace.
Congruence and similarityconcept · mathematics · Cross-eraTwo figures are congruent when one can be placed exactly on the other by moving it without stretching, and similar when one is an exact scaled copy of the other, same shape, possibly different size.
Consequentialismconcept · ethics · Late 20th centuryThe family of moral theories holding that an act is right if and only if it produces the best consequences, and nothing else about it matters.
Conservation of mechanical energyconcept · physics · Early modernWhen only conservative forces act, the sum of kinetic and potential energy stays fixed, letting you compare two moments of motion without tracing the path between them.
Conservatismideology · politics · EnlightenmentNot a blueprint for the good society but a disposition to prefer the tried to the untried, and to reform inheritances rather than replace them.
Constitutional Monarchyideology · politics · Early modernA monarch reigns as head of state within limits fixed by a constitution, while real power rests with an elected government.
Constitutionalismideology · politics · Late 20th centuryThe principle that government power is defined and limited by a fundamental law, standing above ordinary politics and enforced against the state itself.
Constraints, assumptions, and marginsmodel · engineering · Cross-eraA margin is the deliberate gap you leave between what a design must survive and what you actually believe it will face, sized to the uncertainty in what you believe.
Constructivism in International Relationsconcept · politics · Late 20th centuryAnarchy is what states make of it: the interests, threats, and structures of world politics are built out of shared ideas and identities, not fixed by material power alone.
Contractarianismconcept · ethics · Late 20th centuryMorality and politics as the output of a bargain: rules that self-interested rational agents would agree to because each does better inside the agreement than out.
Contractualismconcept · ethics · Late 20th centuryAn act is wrong if it violates any principle that no one could reasonably reject.
Conway's Lawmodel · logic · Late 20th centuryAny organization that designs a system will produce a design whose structure copies the organization's own communication structure.
Coordinates and reference framesconcept · mathematics · Cross-eraA location is meaningless until you fix an origin, a set of directions, and a scale, and describing the same point from a different origin or set of directions gives different numbers for an identical place.
Corporatismideology · politics · Late 20th centurySociety organized not as isolated individuals but as functional bodies (labour, capital, professions) bargaining with each other and the state.
Correlation and Causationconcept · psychology · Late 20th centuryTwo variables moving together tells you they are related, not that one moves the other; only a controlled experiment licenses the word 'cause.'
Council Communismideology · politics · Early 20th centuryThe Marxist current that rejected both the reformist party and the vanguard party, holding that workers' councils are themselves the organs of revolution and of rule.
Counting and cardinalityconcept · mathematics · Cross-eraCounting is a procedure carried out one tag at a time; cardinality is the fact that procedure reveals, that the last tag names the size of the whole group regardless of order.
Creative Destructionconcept · economics · Early 20th centuryCapitalism progresses by destroying what it has built, as new innovations render old industries, firms, and methods obsolete.
Critical Massmodel · logic · Late 20th centuryThe threshold past which a process stops needing a push and starts sustaining itself.
Cross product as oriented area and rotationconcept · mathematics · 19th centuryThe cross product of two vectors is a third vector, perpendicular to both, whose length equals the area of the parallelogram they span and whose direction, fixed by a right hand convention, records the sense of rotation from the first vector to the second.
Crossing the Chasmmodel · economics · Late 20th centuryA discontinuous gap separates the visionaries who first buy a new technology from the pragmatic mainstream, and most products die in it.
Cultural Relativismconcept · ethics · Late 20th centuryThe claim that moral codes are products of particular cultures, and, in its strong form, that no culture's code is truer or better than another's.
Daniel Kahnemanthinker · psychology · Late 20th centuryThe psychologist who mapped the systematic errors of human judgment and, in doing so, helped found behavioral economics.
Daoismtradition · religion · AncientThe way of the Dao: to align with the nameless source of all things by ceasing to force, and to let action arise of itself.
David Humethinker · philosophy · EnlightenmentThe arch-empiricist who showed that our belief in cause, in the self, and in the future rests not on reason but on habit.
David Ricardothinker · economics · 19th centuryThe stockbroker who turned political economy into a deductive science, giving the world the theory of comparative advantage and the most rigorous classical account of distribution.
Defining Abnormalityconcept · psychology · 19th centuryThe prior question beneath any diagnostic manual: what makes a mental state a disorder at all, rather than an eccentricity, a vice, or a life the culture dislikes.
Deliberate Practicemodel · psychology · Late 20th centurySkill grows not from logging hours but from focused, effortful practice at the edge of your ability, guided by immediate feedback.
Demand Characteristicsconcept · psychology · Late 20th centuryParticipants and experimenters read the study's unspoken cues and bend the results toward what they think it is looking for.
Democratic Peace Theoryconcept · politics · EnlightenmentThe most-tested regularity in international relations: liberal democracies almost never go to war with one another, though they fight non-democracies as readily as anyone.
Democratic Socialismideology · politics · Late 20th centurySocialism reached and kept by democratic means: not managing capitalism, but democratizing the economy itself.
Deontologyconcept · ethics · EnlightenmentThe view that some acts are right or wrong in themselves, by their conformity to duty, regardless of their consequences.
Dependency Theoryconcept · economics · Late 20th centuryPoor countries are not simply behind; they are kept poor by the very way they are plugged into the world economy.
Dilution and the Logic of Equityconcept · economics · Late 20th centuryFounders own a shrinking share of a growing company because each round of outside capital issues new stock, and a smaller slice of a larger, better-funded pie can be worth more than a larger slice of a starved one.
Dimensional analysis in physicsmodel · physics · Cross-eraEvery physical quantity carries dimensions built from a small set of base quantities, and any equation claiming to describe nature must have the same dimensions on both sides, which lets you test or repair a formula before ever taking data.
Direct Democracyideology · politics · AncientRule in which citizens make the laws and decide policy themselves, rather than delegating the decision to elected representatives.
Disruptive Innovationmodel · economics · Late 20th centuryIncumbents lose not by managing badly but by managing well: they serve their best customers and cede the low end to cheap, inferior entrants who improve upward and displace them.
Distributismideology · politics · Early 20th centuryA third way between capitalism and socialism that seeks to distribute productive property as widely as possible, so that most people own the means of their own living.
Distributive Justiceconcept · politics · AncientThe question of how a society should allocate its benefits and burdens, and by what standard any pattern of holdings counts as just.
Divine Command Theoryconcept · ethics · AncientThe view that what makes an act morally obligatory is that God commands it, and what makes it wrong is that God forbids it.
The Divine Right of Kingsideology · politics · Early modernThe claim that a king's authority comes straight from God, so he answers to God alone and to no earthly power.
The Division of Laborconcept · economics · EnlightenmentSplitting work into specialized tasks multiplies productivity, but at costs Smith himself foresaw.
The Doctrine of Double Effectconcept · ethics · MedievalAn act with a good and a bad effect can be permissible if the bad effect is foreseen but not intended, is not the means to the good, and is proportionate.
The Dodo Bird Verdictconcept · psychology · Late 20th centuryThe recurring finding that competing psychotherapies, tested head to head, produce roughly equal outcomes, suggesting shared ingredients matter more than technique.
Dot product as alignmentconcept · mathematics · 19th centuryThe dot product of two vectors is a single number, equal to the product of their lengths and the cosine of the angle between them, that measures how much one points along the other.
Double-Loop Learningmodel · psychology · Late 20th centuryErrors can be fixed two ways: by adjusting the action within your existing assumptions, or by questioning the assumptions themselves.
The Drowning Childexperiment · ethics · Late 20th centuryIf you would ruin your suit to save a child drowning in front of you, why does a child dying far away have a weaker claim on you?
Dual-Process Theoryconcept · psychology · Late 20th centuryThe claim that judgment runs on two functionally distinct kinds of mental processing: one fast and automatic, one slow and effortful.
Dunbar's Numbermodel · psychology · Late 20th centuryThere is a rough cognitive ceiling, around 150, on the number of stable social relationships one person can maintain at once.
The Dunning-Kruger Effectconcept · psychology · Late 20th centuryThe claim that the least competent tend to overrate themselves the most, because the skills needed to do well are the same skills needed to judge how well you did.
Duverger's Lawmodel · politics · Late 20th centuryWinner-take-all elections in single-member districts push a polity toward two parties; proportional representation lets many parties survive.
Economic Moatsconcept · economics · Late 20th centuryA structural feature that lets a business keep earning excess returns while competition tries and fails to erode them.
Edmund Burkethinker · politics · EnlightenmentThe first great theorist of conservatism, who argued that inherited institutions embody a wisdom that abstract reason cannot replace.
Edward Tolmanthinker · psychology · Early 20th centuryThe behaviorist who proved rats build "cognitive maps," keeping internal representation alive until neuroscience found it in the brain.
Effectuationmodel · economics · ContemporaryThe decision logic expert entrepreneurs actually use: start from the means you already have, cap your downside at what you can afford to lose, and build the goal with whoever commits, rather than predicting a future and chasing it.
The Eisenhower Matrixmodel · logic · Late 20th centurySort your tasks by importance and urgency, and you will find that the work that matters most is rarely the work that is shouting loudest.
The Elaboration Likelihood Modelmodel · psychology · Late 20th centuryPersuasion travels one of two routes, careful thought about the argument or a shortcut around it, depending on whether the audience is able and motivated to think.
Elastic and inelastic collisionsconcept · physics · Early modernMomentum is conserved in every collision between isolated bodies, but kinetic energy is conserved only in the special case of a perfectly elastic collision.
Emergencemodel · logic · Late 20th centuryHigher-level order arising from many local interactions with no central control, so the whole comes to have properties none of its parts possess.
Emotivismconcept · ethics · Early 20th centuryThe theory that moral statements do not describe facts but express feeling and work on the feelings of others: 'stealing is wrong' means, roughly, 'Boo to stealing.'
Engineering as constrained choiceconcept · engineering · Cross-eraEngineering is the practice of finding the best available choice once physical law, cost, time, safety, and human capability have each ruled out some of what you originally wanted.
The English Schooltradition · politics · Late 20th centuryStates form not merely a system but a society, bound by shared institutions that produce order among them without any government above them.
Entrepreneurial Alertnessconcept · economics · Late 20th centuryEntrepreneurship is not managing resources or bearing risk but being alert to profit opportunities that everyone else has failed to notice.
The Entrepreneurial Ecosystemconcept · economics · 19th centuryEntrepreneurship concentrates in places, not firms: dense local networks of talent, capital, suppliers, and know-how create advantages no isolated company can build alone.
Entropymodel · logic · 19th centuryBecause disordered arrangements vastly outnumber ordered ones, any isolated system tends to slide from order to disorder, and reversing that slide always costs energy.
Equalityconcept · philosophy · ClassicalThe claim that persons are alike in some fundamental respect and that this likeness demands corresponding treatment.
Equality as a balance relationconcept · mathematics · Cross-eraEquality is a claim that two expressions name the same quantity, checkable and preservable under matched changes to both sides, not a command to compute.
Equality of Opportunityconcept · politics · EnlightenmentThe principle that a person's prospects should depend on their own choices and abilities, not on the circumstances of their birth.
Equality of Outcomeconcept · politics · EnlightenmentThe principle that a just society should aim for roughly equal distribution of goods, wealth, or welfare among its members.
Equations as constraintsconcept · mathematics · Cross-eraAn equation is not a puzzle with one ritual answer; it is a filter that sorts every candidate value into allowed or disallowed, and the solution set can be empty, one value, several, or infinite.
Erik Eriksonthinker · psychology · Late 20th centuryErikson mapped human development as eight lifelong crises of identity and relationship, running from an infant's first trust to the reckoning of old age.
Error Theoryconcept · ethics · Late 20th centuryMoral claims try to state objective facts, but there are no objective values, so every positive moral claim is false.
Essence of Decisiontext · politics · Late 20th centuryWhy a state acted depends on which model you use to look at it: a unified rational chooser, a bundle of organizational routines, or a bargain among bureaucratic players.
Estimation and orders of magnitudemodel · mathematics · Cross-eraAn order-of-magnitude estimate breaks an unknown quantity into a chain of roughly known factors, so that the errors in each factor partially cancel and the product lands within a defensible range of the truth.
Eudaimoniaconcept · ethics · ClassicalThe human good is not a feeling of happiness but a whole life lived well, in activity that expresses excellence of character and mind.
The Euthyphro Dilemmaexperiment · ethics · ClassicalIs the good good because God commands it, or does God command it because it is good? Either answer costs the believer something.
Executive Functionsconcept · psychology · Late 20th centuryThe mind's top-down control system: the processes that hold a goal in mind, suppress the automatic response, and adjust when the situation changes.
Existentialismtradition · philosophy · 19th centuryThere is no fixed human essence handed down in advance; we exist first and then, through free choice, make ourselves what we are, and must invent our own meaning in a world that supplies none.
Expected Valuemodel · logic · Early modernThe value of an uncertain prospect is each possible outcome multiplied by its probability, all added together: the long-run average of the gamble.
The Experience Machineexperiment · ethics · Late 20th centuryNozick's machine offers a lifetime of perfect experiences; most people refuse it, which suggests we want more from life than how it feels from the inside.
Exposure and Extinctionconcept · psychology · Early 20th centuryConditioned fear weakens when the feared thing is faced without the expected harm, and avoidance is what keeps the fear alive.
Externalitiesconcept · economics · Early 20th centuryAn externality is a cost or benefit that falls on someone who had no say in the transaction that produced it.
Failure modes and graceful degradationmodel · engineering · Cross-eraA failure mode is one specific way a function can stop doing what it is supposed to do, and graceful degradation is designing so that when a failure mode occurs, the system loses capability in a controlled way rather than becoming dangerous all at once.
Falsificationismconcept · logic · Early 20th centuryA theory is scientific only if it forbids something observable and could in principle be proven wrong; science grows by bold guesses and hard attempts to refute them.
Fascismideology · politics · Late 20th centuryA revolutionary ultranationalism that promises a nation's rebirth through a total state, a supreme leader, and the cult of violence, and that in power delivered dictatorship and war.
Fasteners and jointsconcept · engineering · Cross-eraEvery joint, bolted, welded, riveted, glued, press fit, or snapped together, is a deliberate choice about how a load path crosses from one part into another, and the choice trades off strength, cost, and whether the joint can be made, inspected, and undone again.
Federalismideology · politics · EnlightenmentA single country in which sovereignty is divided by a written constitution between a central government and constituent units, each supreme within its own sphere.
Feedback Loopsmodel · logic · 19th centuryA feedback loop is a circle of cause and effect in which a system's output loops back to change its own input, either damping the change (balancing) or amplifying it (reinforcing).
Feminist International Relationstradition · politics · Late 20th centuryThe claim that international relations theory only looks universal because it hides the women who sustain it and mistakes a masculine account of power for the whole of world politics.
Feudalismideology · politics · MedievalThe medieval European order in which land was held in return for service, binding lords, vassals, and laboring peasants in a chain of reciprocal obligation.
The Feynman Techniquemodel · logic · Late 20th centuryTo understand something, explain it in plain words as if to a beginner; where you stall or reach for jargon, you have found what you do not actually know.
First Principles Thinkingmodel · logic · AncientReason up from the few things you are sure are true, rather than down from what everyone already assumes.
First-Wave Feminismideology · politics · 19th centuryThe movement to win women's legal and political equality, above all the vote and the right to hold property.
Flowmodel · psychology · Late 20th centuryThe state of absorbed, effortless concentration that arises when a hard task and a matched skill meet, so that action and awareness fuse and the activity becomes worth doing for its own sake.
The Flywheelmodel · economics · ContemporaryMomentum is built turn by turn: no single push transforms anything, but consistent effort in one direction compounds into force that becomes nearly unstoppable.
Force as interactionconcept · physics · Early modernA force is not a substance an object carries; it is what one object does to another during an interaction, and every force names a specific pair and a specific cause.
Fractions as numbers, ratios, and operatorsconcept · mathematics · Cross-eraA fraction is a single number, the answer to a division that whole numbers alone cannot finish, and its many everyday faces, a part of a whole, a ratio, a scaling operator, a point on a line, all name that same number.
The Frankfurt Schooltradition · philosophy · Early 20th centuryA tradition of Marxist social theory that turned from economics to culture, asking why reason and enlightenment produced domination rather than freedom.
Free Will and Determinismconcept · philosophy · AncientWhether the choices we experience as free can be real if every event, including the choice, is fixed by prior causes and the laws of nature.
Friedrich Engelsthinker · politics · 19th centuryCo-architect of Marxism, empirical chronicler of industrial capitalism, and the thinker who shaped how the world received Marx's ideas.
Friedrich Hayekthinker · economics · Late 20th centuryThe economist and philosopher who argued that dispersed knowledge makes central planning impossible and that freedom depends on the rule of law, not the rule of planners.
Friedrich Nietzschethinker · philosophy · 19th centuryThe philosopher who diagnosed the death of God and asked what humanity could become once it stopped pretending its values came from anywhere but itself.
Functional decompositionmodel · engineering · Cross-eraFunctional decomposition is breaking a desired outcome into the smaller functions that must all happen for it to occur, described by what each function does rather than by what part will do it.
Functions as machines and relationshipsconcept · mathematics · Cross-eraA function is a rule that assigns to every element of an allowed set of inputs exactly one element of an output set, regardless of whether the rule is a formula, a table, or a picture.
The Fundamental Attribution Errorconcept · psychology · Late 20th centuryThe tendency, when judging others, to blame their character and discount their circumstances.
G.W.F. Hegelthinker · philosophy · 19th centuryThe philosopher who argued that reality, mind, and history unfold through contradiction toward the self-knowledge of a single Spirit.
Gall's Lawmodel · logic · Late 20th centuryA complex system that works is invariably found to have evolved from a simple system that worked; you cannot build a working complex system from scratch.
Game Theorymodel · economics · Early 20th centuryThe mathematics of strategic decisions, where the best thing to do depends on what everyone else is deciding to do.
The Garcia Effectexperiment · psychology · Late 20th centuryRats made sick hours after tasting sweet water learned to avoid the sweetness alone, a result that broke the rule that any cue can be conditioned to any consequence.
Gate Control Theory of Painmodel · psychology · Late 20th centuryPain is not a direct readout of injury but a signal a spinal gate can amplify or dampen, which is why rubbing a hurt helps and why a lost limb can still ache.
Gears, belts, chains, and transmissionsconcept · engineering · Cross-eraA gear, belt, or chain transmission trades rotational speed for torque, or torque for speed, in an exact ratio set by the size of the wheels it connects.
The General Willconcept · politics · EnlightenmentThe general will is the collective interest of a political community, distinct from the mere sum of private desires, and the only legitimate basis of sovereign authority.
Georgismideology · economics · 19th centuryThe value of land is created by the community, not the owner, so it should be taxed away and used for public needs, while labor and capital are left untaxed.
Gestalt Principles of Perceptionconcept · psychology · Early 20th centuryThe mind groups raw sensory fragments into organized wholes by rules like proximity, similarity, closure, and continuity, rather than building perception up piece by piece.
Gestalt Psychologytradition · psychology · Early 20th centuryThe claim that perception and thought are organized into structured wholes that cannot be built up from separate sensory parts.
The Gettier Problemexperiment · logic · Late 20th centuryA short 1963 paper showed that a belief can be true, and justified, and still not count as knowledge, breaking a definition that had stood since Plato.
The Globalization Trilemmamodel · economics · ContemporaryDeep economic integration, the nation-state, and democracy cannot all hold at once: pick any two and you must give up the third.
Goal-Setting Theoryconcept · psychology · Late 20th centurySpecific, difficult goals produce higher performance than vague or easy ones, through named mechanisms and inside strict limits that, when ignored, make the same tool backfire.
The Golden Meanconcept · ethics · ClassicalEach virtue of character is a mean between two vices, one of excess and one of deficiency, and hitting it takes practical wisdom, not arithmetic.
Goodhart's Lawmodel · logic · Late 20th centuryOnce you turn a measurement into a target, people optimize the measurement rather than the thing it was meant to track, and the measurement goes bad.
Graphs as pictures of relationshipsconcept · mathematics · Cross-eraA graph is a picture in which position stands for a fact, so shape, slope, and turning points are frozen statements about how one quantity depends on another.
Green Political Theoryideology · politics · Late 20th centuryPolitics reorganized around the recognition that the economy sits inside a finite biosphere, and that nature and future generations have standing.
Groupthinkconcept · psychology · Late 20th centuryIrving Janis's theory that tight-knit, high-esteem groups under stress suppress dissent to preserve unanimity, and decide worse for it.
Hanlon's Razormodel · logic · Late 20th centuryNever attribute to malice that which is adequately explained by stupidity, carelessness, or incompetence.
Hannah Arendtthinker · philosophy · Late 20th centuryA political thinker who insisted that the capacity to act freely in public, not economics or history, is the core of political life.
The Hard Problem of Consciousnessconcept · philosophy · Late 20th centuryWhy does any physical process in the brain feel like something from the inside, rather than going on in the dark?
The Harm Principleconcept · politics · 19th centuryThe only purpose for which power can rightfully be exercised over anyone against their will is to prevent harm to others, never for their own good.
Hedonismconcept · ethics · AncientThe claim that pleasure, or the absence of pain, is the only thing good in itself.
Hegemonyconcept · politics · ClassicalHegemony is the dominance of one group over others, secured less by force than by making the ruling order appear natural and inevitable.
How Memory Worksconcept · psychology · 19th centuryMemory is not a single faculty but a set of processes, encoding, storage, and retrieval, each of which can fail, and forgetting is what happens when one of them does.
Human Rightsconcept · ethics · EnlightenmentMoral claims that belong to every human being simply because they are human, prior to and independent of any government's grant.
Humanistic Psychologytradition · psychology · Late 20th centuryA 'third force' in psychology, founded by Maslow and Rogers against behaviorism and psychoanalysis, that studies growth, meaning, and the healthy personality rather than conditioned response or pathology.
Hypnosisconcept · psychology · 19th centuryA procedure that uses suggestion to produce marked changes in perception, memory, and voluntary action in willing, highly suggestible people, whose reality and mechanism are still contested.
Hypothetical Imperativesconcept · ethics · EnlightenmentCommands that bind you only if you want something: if you want the end, take the means. Kant argues morality cannot be one of them.
Idiographic versus Nomothetic Approachesconcept · psychology · 19th centuryThe split between studying general laws that hold across people and understanding the single individual in depth.
Illiberal Democracyideology · politics · ContemporaryA system that keeps the vote but strips out the limits: elections continue while the free press, the courts, minority rights, and the rule of law are hollowed from within.
Immanuel Kantthinker · philosophy · EnlightenmentThe philosopher who argued that the mind actively shapes experience, and that morality is a law reason gives to itself.
Implicit Biasconcept · psychology · Late 20th centuryThe idea that people hold automatic, unendorsed associations that can bias judgment, and the hard, unresolved fight over whether the test that made it famous actually measures or predicts anything.
The Impossible Trinitymodel · economics · Late 20th centuryA country can have a fixed exchange rate, free capital movement, and independent monetary policy, but only two of the three at once.
Impulse and force over timeconcept · physics · Early modernThe same change in momentum can come from a large force acting briefly or a small force acting longer, since impulse, force multiplied by the time it acts, is what equals the change in momentum.
Incentivesmodel · economics · Cross-eraPeople do what they are rewarded for doing, so the reward you set, not the outcome you wanted, is what you will actually get.
Individualism and Collectivismconcept · psychology · ContemporaryThe most-studied axis in cross-cultural psychology, on which cultures range from prizing the self-contained individual to prizing the person embedded in a group.
Individualist Anarchismideology · politics · 19th centuryThe strand of anarchism that begins from the sovereign individual and refuses any authority, collective or state, that the individual has not chosen.
Inertia and the persistence of motionconcept · physics · Cross-eraInertia is the tendency of an object to keep whatever velocity it already has, constant speed in a constant direction, unless something acts on it to change that velocity, which means force is needed to change motion, not to maintain it.
Insight Learningexperiment · psychology · Early 20th centuryWolfgang Köhler's chimpanzees solved problems by a sudden reorganization of the whole situation, not by gradual trial and error, and that challenged the behaviorist account of how learning works.
Internal and External Validityconcept · psychology · Late 20th centuryTwo questions every study faces: can we trust its causal claim on its own terms, and does that claim hold outside the study?
Inversionmodel · logic · 19th centuryTo solve a hard problem, turn it around: instead of asking how to succeed, ask what would guarantee failure, and then refuse to do it.
The Invisible Handconcept · economics · EnlightenmentThe idea that self-interested action, channeled through competitive markets, can generate social benefit without anyone planning it.
The Is-Ought Problemconcept · ethics · EnlightenmentHume's observation that no statement about what ought to be done follows logically from statements about what merely is.
Jean Piagetthinker · psychology · Early 20th centuryThe psychologist who mapped childhood into stages, arguing that a child's mind is not a small adult's but a different kind of reasoning machine built anew at each age.
Jean-Jacques Rousseauthinker · politics · EnlightenmentThe philosopher who argued that human beings are naturally good but corrupted by society, and that legitimate political authority rests on the collective self-rule of citizens through the general will.
Jeremy Benthamthinker · philosophy · EnlightenmentThe philosopher who made happiness measurable and insisted that law and morality serve nothing higher than human (and animal) well-being.
Jobs to Be Donemodel · economics · ContemporaryCustomers do not buy products, they hire them to make progress on a job that arises in a specific circumstance.
The Johari Windowmodel · psychology · Late 20th centuryA four-pane map of the self, split by what you know about yourself and what others know about you, used to widen the shared, honest space between people.
John Kenneth Galbraiththinker · economics · Late 20th centuryThe economist who argued that affluent capitalism's deepest failure is not scarcity but the misdirection of abundance toward private goods and away from public needs.
John Lockethinker · philosophy · Early modernThe philosopher who grounded government in consent and individual rights, shaping the liberal tradition more than any other single thinker.
John Maynard Keynesthinker · economics · Early 20th centuryThe economist who argued that markets can fail catastrophically and that governments must act to restore employment.
John Rawlsthinker · politics · Late 20th centuryThe philosopher who revived political philosophy by asking how free and equal people could agree on the terms of a just society.
John Stuart Millthinker · philosophy · 19th centuryThe philosopher who inherited utilitarianism and liberalism, then remade both: pleasures have quality as well as quantity, and the only warrant for coercing an adult is to prevent harm to others.
Joseph Schumpeterthinker · economics · Early 20th centuryThe economist who saw capitalism as a system defined not by equilibrium but by the perpetual upheaval of creative destruction.
Just War Theorytradition · politics · AncientA tradition that treats war as sometimes justified but always bound by rules: rules for when you may fight, and rules for how you may fight.
Justiceconcept · philosophy · ClassicalThe attempt to answer what people are owed and why, and to build institutions that deliver it.
Karen Horneythinker · psychology · Early 20th centuryThe neo-Freudian analyst who moved neurosis from instinct to culture and relationships, and who dismantled Freud's account of women from inside psychoanalysis.
Karl Marxthinker · philosophy · 19th centuryThe thinker who recast history as a struggle between economic classes and diagnosed capitalism as a self-undermining system built on exploited labor.
Keynesianismideology · economics · Early 20th centuryThe claim that total spending, not thrift, drives output and jobs, so government can and should steady the economy when private demand fails.
Kinetic energyconcept · physics · Early modernKinetic energy is the energy a moving object carries by virtue of its motion, equal to one half its mass times the square of its speed, derived directly from the work needed to accelerate it from rest.
Kinetic friction and energy lossconcept · physics · EnlightenmentKinetic friction is a roughly constant force opposing sliding motion that converts organized kinetic energy into heat at the contact interface, at a rate set by the normal force and a material coefficient.
The Kingdom of Endsconcept · ethics · EnlightenmentAct as if you were both lawmaker and citizen in a community where every rational being is treated as an end and never merely as a means.
Knightian Uncertaintyconcept · economics · Early 20th centuryThe line between risk, which has a knowable probability distribution and can be insured against, and true uncertainty, which is unique and unmeasurable, and which is the source of genuine entrepreneurial profit.
Kohlberg's Stages of Moral Developmentconcept · psychology · Late 20th centuryA six-stage account of how moral reasoning matures, tested through dilemmas like Heinz and the drug he cannot afford, and famously challenged for treating one voice, justice, as the summit of maturity.
The Labor Theory of Valueconcept · economics · Early modernThe claim that the economic value of a commodity is determined by the quantity of socially necessary labor required to produce it.
The Lean Startupmodel · economics · ContemporaryTreat a startup as an experiment engine whose real product is validated learning, and test cheap falsifiable hypotheses before you scale.
Learned Helplessnessexperiment · psychology · Late 20th centuryRepeated exposure to a shock a dog cannot escape teaches it, wrongly, that nothing it does will change what happens, even later when escape is easy.
Legal Positivismconcept · philosophy · 19th centuryThe view that whether something is law is a matter of social fact, not moral merit: legal validity and moral worth are separate questions.
Legitimacyconcept · politics · ClassicalLegitimacy is the quality that makes a political authority rightful rather than merely powerful.
Length, area, and volumeconcept · mathematics · Cross-eraLength, area, and volume are all the same operation, counting how many unit pieces tile a shape, applied in one, two, and three dimensions respectively, and every area or volume formula is a shortcut for that count.
Leninismideology · politics · Early 20th centuryMarxism rebuilt around a disciplined revolutionary party that seizes state power in capitalism's weakest link and rules in the name of the working class.
Lev Vygotskythinker · psychology · Early 20th centuryThe Soviet psychologist who argued that the higher mind is built from the outside in, through social interaction and cultural tools, before it becomes private thought.
Levels of Explanationconcept · psychology · Late 20th centuryAny information-processing system can be described at three separate levels (what problem it solves, by what procedure, and in what physical stuff), and picking the wrong level guarantees the wrong explanation.
The Levels-of-Analysis Problemconcept · politics · Late 20th centuryThe same international event can be explained at the level of the individual, the state, or the system, and choosing a level is a prior commitment that decides what will even count as a cause.
Leveragemodel · economics · Cross-eraA small, well-placed input that moves a load far larger than itself, magnifying the result in both directions.
Levers, linkages, cams, and motion conversionconcept · engineering · Cross-eraA lever, linkage, or cam uses pure geometry, not motors or electronics, to transform a given force or motion into a different force or a different, deliberately chosen path.
The Leviathanconcept · politics · Early modernA commonwealth so powerful that it can overawe all its members, created by their own agreement because the alternative is a war of all against all.
Liberal Democracyideology · politics · Late 20th centuryMajority rule fenced in by individual rights: the people choose their rulers, but the rulers cannot do just anything the majority wants.
Liberalism in International Relationstradition · politics · EnlightenmentThe claim that anarchy among states can be tamed: trade, shared republican values, and international institutions make cooperation and even peace possible where realism sees only recurring conflict.
Libertarianismideology · politics · Late 20th centuryThe political philosophy that liberty is the highest value, that individuals own themselves, and that the state must be kept as small as protecting those rights allows.
Libertyconcept · philosophy · ClassicalLiberty is the condition in which a person is not subject to interference, domination, or constraint by others in matters that are properly their own.
The Lindy Effectmodel · logic · Late 20th centuryFor things that do not age, the longer they have already survived, the longer they can be expected to survive: age is evidence of durability.
Little Albertexperiment · psychology · Late 20th centuryThe 1920 study in which two psychologists conditioned an infant to fear a white rat, a landmark of behaviorism and one of the most ethically indefensible experiments in the field's history.
Loads, load paths, and free-body thinkingconcept · engineering · Cross-eraA load path is the actual route a force travels, member by member, from where it is applied to the ground or support that finally absorbs it, and free-body thinking is the method of isolating one member at a time to check whether that route can carry what is asked of it.
Localization of Functionconcept · psychology · Late 20th centuryThe claim that specific mental functions are carried out by specific, identifiable parts of the brain.
The Long Tailmodel · economics · ContemporaryWhen shelf space is unlimited, the many niche products can collectively rival the few hits.
Loss Aversionconcept · psychology · Late 20th centuryLosses hurt more than equivalent gains feel good, so people fight harder to avoid losing than to win the same amount.
Ludwig Wittgensteinthinker · philosophy · Early 20th centuryThe philosopher who twice tried to dissolve philosophy itself: first by drawing the limits of what language can picture, then by showing that meaning is not picturing at all but use.
Mahayana Buddhismtradition · religion · AncientThe 'Great Vehicle' of Buddhism, which holds that the goal is not private liberation but the vow to free all beings, grounded in the doctrine that all things are empty of fixed self-nature.
Maoismideology · politics · Late 20th centuryA revolution built not on the industrial worker but on the poor peasant, seized through protracted guerrilla war and kept permanently boiling by mass campaigns from above.
The Map Is Not the Territorymodel · logic · Early 20th centuryA representation of reality is never the reality itself, and mistaking the two is a basic and costly error.
Margin of Safetymodel · economics · Early 20th centuryBuy a thing for meaningfully less than it is worth, so that error, bad luck, and the unforeseen cannot ruin you.
Market Socialismideology · politics · Late 20th centuryKeep the market as the way goods get allocated, but move the ownership of firms out of private hands and into workers or the public.
Marxismideology · politics · 19th centuryA theory that history is driven by class struggle over the means of production, that capitalism exploits labor and sows the seeds of its own collapse, and that this points toward a classless society.
Marxism-Leninismideology · politics · Late 20th centuryThe official ideology of the Soviet state: Marx's theory of history refitted with Lenin's vanguard party and codified under Stalin into a doctrine of one-party rule.
Mary's Roomexperiment · philosophy · Late 20th centuryA scientist who knows every physical fact about color but has only seen black and white: when she first sees red, does she learn something new, and if so, is physicalism false?
Maslow's Hierarchy of Needsmodel · psychology · Early 20th centuryHuman motivation is organized into tiers, with basic bodily and safety needs generally pressing before higher needs for belonging, esteem, and self-actualization.
Mass as resistance to accelerationconcept · physics · Early modernMass is not how much stuff an object has or how big it looks; it is a measured resistance, how much acceleration a given force fails to produce.
The Median Voter Theoremmodel · politics · Late 20th centuryUnder majority rule on a single issue dimension, vote-seeking candidates are pulled toward the position of the median voter.
Mercantilismideology · economics · Early modernThe early-modern doctrine that a nation grows rich by hoarding gold and silver through trade surpluses, protected tariffs, and state-chartered monopolies.
The Mere Addition Paradoxexperiment · ethics · Late 20th centuryParfit's step-by-step trap: adding happy people is not bad, and equalizing while raising welfare is good, yet repeating these steps proves a huge population of barely worthwhile lives is best.
Meritocracyideology · politics · Late 20th centuryThe rule that positions and rewards should go to the able and the hardworking rather than to the well born or the well connected.
Metacognitionconcept · psychology · Late 20th centuryThinking about your own thinking: the capacity to monitor what you know and to steer your own learning on the strength of that judgment.
Michel Foucaultthinker · philosophy · Late 20th centuryThe philosopher who argued that power does not merely repress us but produces us, manufacturing the subjects, truths, and normalities we take for granted.
Mikhail Bakuninthinker · politics · 19th centuryThe father of revolutionary anarchism, who made the destruction of the state the center of radical politics.
Milgram's Obedience Experimentsexperiment · psychology · Late 20th centuryThe lab studies that found most ordinary people will keep delivering apparent electric shocks to a stranger simply because a man in a lab coat tells them to continue.
Milton Friedmanthinker · economics · Late 20th centuryThe economist who made the case that free markets and sound money, not government planning, are the surest path to broad prosperity.
Minarchismideology · politics · Late 20th centuryThe state should do only what protects rights: police, courts, and defense, and nothing more.
Modern Liberalismideology · politics · 19th centuryThe liberalism that kept individual liberty as its goal but decided that real freedom needs a material floor, and so accepted the welfare state and the mixed economy.
Modern Monetary Theoryideology · economics · ContemporaryA government that issues its own currency can never run out of it, so the real limit on public spending is inflation and idle resources, not the risk of going broke.
Modernization Theorymodel · politics · Late 20th centuryThe claim that the richer a country becomes, the more likely it is to become and stay a democracy.
Momentum as quantity of motionconcept · physics · Early modernMomentum, mass times velocity, is conserved for an isolated system because every internal force comes paired with an equal and opposite reaction acting for the same time.
Monetarismideology · economics · Late 20th centuryMoney drives prices: control the growth of the money supply by a steady rule and you tame inflation, where discretionary fine-tuning fails.
Montesquieuthinker · politics · EnlightenmentThe thinker who argued that political liberty depends on the structural separation of governmental powers.
Moral Anti-Realismconcept · ethics · Late 20th centuryThe family of views denying that there are objective moral facts: our moral talk expresses attitudes, constructs standards, or is systematically false.
Moral Hazardmodel · economics · 19th centuryWhen you are shielded from the downside of a risk, you take more of it than you otherwise would, because someone else pays if it goes wrong.
Moral Intuitionismconcept · ethics · Late 20th centuryThe view that some moral truths are self-evident, known directly by careful reflection rather than derived from anything more basic.
Moral Luckconcept · ethics · Late 20th centuryFactors no one controls, from the swerve of a child into the road to the temperament you were born with, shape how much blame and praise a person actually deserves.
Moral Nihilismconcept · ethics · Late 20th centuryThe view that there are no moral facts, so no action is really right or wrong.
Moral Realismconcept · ethics · Late 20th centuryThe view that there are objective moral facts, true or false independently of what anyone thinks or feels.
Moral Relativismconcept · ethics · Late 20th centuryThe view that moral truth is not absolute but relative to a culture or an individual, so that no moral framework is objectively privileged over the others.
Motion graphsconcept · physics · Cross-eraPosition time, velocity time, and acceleration time graphs are three pictures of the same motion, connected because slope on one graph gives the value plotted on the next, and area under one gives the change plotted on the graph before it.
Murray Rothbardthinker · economics · Late 20th centuryThe economist and philosopher who pushed libertarianism to its logical extreme, arguing that every function of the state could and should be replaced by voluntary market arrangements.
Mutualismideology · politics · 19th centuryA stateless economy of independent producers who trade as equals, own only what they use, and lend to one another at cost.
The Nash Equilibriummodel · economics · Late 20th centuryA situation in which no player can do better by unilaterally changing their own strategy, given what everyone else is doing.
National Conservatismideology · politics · ContemporaryA contemporary right that fuses cultural traditionalism with national sovereignty, set against globalism and rule by experts.
Nationalismideology · politics · EnlightenmentThe claim that humanity is divided into nations, that the nation is the primary object of political loyalty, and that each nation should govern itself.
Natural Lawconcept · ethics · AncientThe view that there is an objective moral order grounded in human nature and knowable by reason, prior to and independent of any human command.
Natural Rightsconcept · philosophy · Early modernThe claim that certain rights belong to every person by nature, prior to any government or law.
Natural Selectionmodel · logic · 19th centuryWhere things copy imperfectly and copies compete, the versions that survive and reproduce best come to prevail, and adaptation accumulates with no one designing it.
The Naturalistic Fallacyconcept · ethics · Early 20th centuryMoore's charge that any attempt to define the property good in terms of a natural property (pleasure, desire, fitness) mistakes what good is.
Nature versus Nurtureconcept · psychology · 19th centuryThe old contest between biology and upbringing dissolves once behavioral genetics shows that genes and environment do not add, they interact.
Nazismideology · politics · Early 20th centuryA German racial and genocidal variant of fascism that built a total state to wage a war of conquest and to murder Europe's Jews.
The Need to Belongconcept · psychology · Late 20th centuryThe drive to form and keep lasting, caring bonds is a basic human need whose frustration harms mind and body.
Negative numbers as direction and debtconcept · mathematics · Cross-eraA negative number is the answer that must exist if subtraction is to always make sense, and it names an opposite, in direction or in owed amount, rather than a smaller kind of nothing.
Negative Utilitarianismconcept · ethics · Late 20th centuryThe view that reducing suffering matters more than promoting happiness, or matters exclusively.
Neoliberal Institutionalismconcept · politics · Late 20th centurySelf-interested states, though they answer to no world government, can still cooperate because institutions cut the costs and risks of dealing with one another.
Neoliberalismideology · politics · Late 20th centuryThe twentieth-century revival of market liberalism: free markets, free trade, sound money, and a state that clears the way rather than steers the economy.
Network Effectsmodel · economics · Late 20th centuryA product grows more valuable to each user as more people use it, which lets the leader pull away and often take most of the market.
Neuroplasticityconcept · psychology · 19th centuryThe brain's lifelong capacity to reorganize its own structure and connections in response to experience, injury, and use.
The Norm Life Cyclemodel · politics · Late 20th centuryA three-stage account of how a moral standard goes from the claim of a few activists to a rule almost no one questions: emergence, cascade, internalization.
The Normal Distributionmodel · logic · EnlightenmentThe bell curve: a model of how quantities scatter when they are the sum of many small independent effects, reliable at the center and dangerous when its thin tails are trusted where they do not belong.
Normal force and contact constraintsconcept · physics · Early modernThe normal force is the perpendicular push a rigid surface supplies, adjusted to whatever value stops an object from passing through it, not a fixed force equal to weight.
Nuclear Deterrenceconcept · politics · Late 20th centuryThe paradox that a state can be made safer by holding open the certain capacity to destroy an attacker, so that no attack is worth launching.
Nudgeconcept · politics · ContemporaryBecause how choices are arranged always tilts them one way, arrange the defaults and framing to help people while leaving them free to opt out.
Observation, measurement, and physical quantitiesconcept · physics · Cross-eraA physical quantity is not a private sensation but whatever a stated, repeatable procedure of comparison assigns a number and a unit to.
Observational Learningexperiment · psychology · Late 20th centuryBandura's Bobo doll studies showed that children learn new behavior, including aggression, simply by watching a model perform it and seeing what happens next.
Occam's Razormodel · logic · MedievalAmong competing explanations that account for the same facts, prefer the one that assumes the least.
Oligarchyideology · politics · AncientRule by the few, most often the wealthy, exercised in their own interest rather than the common good.
One-Nation Conservatismideology · politics · 19th centuryA conservatism that holds the propertied classes to a duty of care for the poor, so that the nation does not fracture into two.
The Open Question Argumentconcept · ethics · Early 20th centuryMoore's test: if 'is X good?' still feels like a real question, then 'good' cannot simply mean X.
Operant Conditioningexperiment · psychology · Early 20th centuryBehavior that is rewarded tends to recur, behavior that is punished or ignored tends to fade, and Skinner built a whole science out of that one fact.
Opportunity Costmodel · economics · 19th centuryThe true cost of any choice is the value of the best alternative you gave up to make it.
The Original Positionconcept · politics · Late 20th centuryThe imagined choosing situation, behind the veil of ignorance, in which free and equal parties pick the principles that will govern their society.
P-Hacking and HARKingconcept · psychology · Late 20th centuryTwo ways to manufacture a discovery that is not there: keep reshaping the analysis until it clears the significance bar (p-hacking), or write the hypothesis after you already know the answer (HARKing).
Paradigms and Scientific Revolutionsconcept · logic · Late 20th centuryScience does not grow by steady accumulation but by long stretches of settled puzzle-solving punctuated by revolutions that overthrow the framework itself.
The Paradox of Votingconcept · politics · Late 20th centuryIf one vote almost never decides an election, a purely self-interested person should not bother to vote, yet millions do.
The Pareto Principlemodel · economics · 19th centuryA minority of causes usually accounts for a majority of effects, so effort spent on the vital few beats effort spread across the trivial many.
Parkinson's Lawmodel · economics · Late 20th centuryWork expands to fill the time available for its completion, and bureaucracies grow of their own accord regardless of the work to be done.
Patterns, sequences, and rulesconcept · mathematics · Cross-eraA pattern is not proven by noticing repetition in a few terms; it is established by stating a rule that generates every term, and a short list is compatible with more than one such rule.
The Person-Situation Debateconcept · psychology · Late 20th centuryWalter Mischel's demonstration that personality traits predict behavior far more weakly across situations than assumed, and the interactionist synthesis it eventually produced.
Peter Kropotkinthinker · politics · 19th centuryThe anarchist prince who argued that cooperation, not competition, is the primary engine of evolution and the natural basis for a stateless society.
The Peter Principlemodel · economics · Late 20th centuryIn a hierarchy, people are promoted until they reach a job they cannot do, and then they stay there.
Phenomenologytradition · philosophy · Early 20th centuryThe disciplined description of experience exactly as it is lived, from the first-person standpoint, before any theory is laid over it.
Phineas Gage and H.M.concept · psychology · 19th centuryTwo brain-damaged patients, one who lost his temperament and one who lost his ability to form memories, who taught neuroscience both what a single case can prove and how easily its story gets embellished in the retelling.
Phronesisconcept · ethics · ClassicalThe practical wisdom to see what a particular situation calls for and to act well in it, which no rulebook can fully replace.
Physical models and idealizationconcept · physics · Cross-eraA physical model is a deliberately simplified stand in for a real system, built by naming and discarding certain features on purpose, and it is only useful once you can state what it ignores and where that neglect starts to matter.
Pierre-Joseph Proudhonthinker · politics · 19th centuryThe first person to call himself an anarchist, who argued that property is theft, government is tyranny, and free association among equals is the only legitimate basis for social order.
Pirate Metrics (AARRR)model · economics · ContemporaryThe customer lifecycle broken into five measurable stages, Acquisition, Activation, Retention, Referral, Revenue, so you fix the one leaking stage instead of chasing a flattering total.
Place value and number basesconcept · mathematics · Cross-eraPlace value lets a fixed, small set of symbols represent any quantity, however large, by letting a digit's position multiply its meaning.
The Placebo Effectconcept · psychology · Late 20th centuryThe measurable change in a person that follows an inert treatment, driven not by the treatment but by expectation, conditioning, and the ritual of being cared for.
Platothinker · philosophy · ClassicalThe Athenian who turned philosophy into a written art and argued that behind the shifting world lies a realm of perfect, unchanging Forms grasped only by reason.
Plutocracyideology · politics · Cross-eraRule by the wealthy: a system in which money translates, directly or indirectly, into political power.
Points, lines, angles, and planesconcept · mathematics · Cross-eraGeometry builds everything, triangles, circles, solids, from a small set of undefined primitives, point, line, and plane, plus a short list of relations, incidence, betweenness, and angle, that constrain how those primitives can meet.
The Policy Windowmodel · politics · Late 20th centuryPolicy change happens when three separate streams (problems, policies, and politics) are joined by an entrepreneur during a brief window of opportunity.
Political Realismtradition · politics · ClassicalThe tradition that treats international politics as a struggle for power among states in a world with no ruler above them, where survival, not virtue, is the first law.
Populismideology · politics · Late 20th centuryA thin politics that splits society into a virtuous people and a corrupt elite, and claims to speak for the people alone.
Porter's Five Forcesmodel · economics · Late 20th centuryAn industry's long-run profitability is decided by five structural forces, and strategy is the search for a position where those forces are weak and defensible.
Porter's Generic Strategiesmodel · economics · Late 20th centuryCompetitive advantage comes from being the low-cost producer or being different, aimed either broadly or at a niche; try to be everything and you end up stuck in the middle.
Position, distance, and displacementconcept · physics · Cross-eraPosition is where something is relative to a chosen origin, distance is the total length of the path travelled, and displacement is the straight, directed change between start and end, and the three are related but never interchangeable.
Positive and Negative Libertyconcept · politics · Late 20th centuryTwo rival conceptions of freedom: freedom from interference and freedom to live a self-directed life.
Positive Psychologytradition · psychology · Late 20th centurySeligman's 1998 call to study what makes life worth living, not only what goes wrong with it, built into a research program on character, resilience, and gratitude that later had to reckon with its own weak replications.
Postcolonialismtradition · politics · Late 20th centuryThe study of how colonialism reshaped culture, knowledge, and identity, and how those distortions outlasted the empires that produced them.
Potential energy and configurationconcept · physics · Early modernPotential energy is the work a configuration of interacting objects could still do, stored not in one object but in the relationship between them and a reference.
Powerconcept · politics · ClassicalPower is the capacity to shape outcomes, whether by coercion, consent, or the invisible structuring of what counts as thinkable.
Power Laws and Fat Tailsmodel · logic · Late 20th centuryIn a power law a handful of events account for most of the total, the distribution has no typical size, and the rare extreme, not the average, is what dominates the outcome and the risk.
Pragmatismtradition · philosophy · Late 20th centuryThe meaning and truth of an idea lie in its practical consequences: a belief is a rule for action, and inquiry, not correspondence to a fixed reality, is where knowledge is settled.
Precision, accuracy, and uncertaintyconcept · physics · Cross-eraPrecision is how repeatable a measurement is, accuracy is how close it is to the true value, and uncertainty is the honestly stated range within which the true value probably lies.
Preference Utilitarianismconcept · ethics · Late 20th centuryDo the most good by satisfying the most preferences, not by maximizing pleasant feelings.
Prescriptivismconcept · ethics · Late 20th centuryMoral judgments are not reports or mere expressions of feeling but universal commands: to call an act wrong is to prescribe against it for every relevantly similar case, including your own.
Presidentialism versus Parliamentarismconcept · politics · Late 20th centuryThe comparative-institutions argument over whether the executive should stand on its own popular mandate and fixed term (presidentialism) or emerge from and answer to the legislature (parliamentarism).
The Principal-Agent Problemmodel · economics · Late 20th centuryWhenever one person acts on another's behalf but wants different things and cannot be fully watched, their interests drift apart, and closing the gap is never free.
The Prisoner's Dilemmaexperiment · logic · Late 20th centuryTwo rational actors, each acting in their own interest, reach an outcome that is worse for both than if they had cooperated.
Probabilistic Thinkingmodel · logic · Late 20th centuryTreat beliefs as probabilities to be sized and updated, not as certainties to be defended.
The Problem of Evilconcept · philosophy · AncientIf God is all-powerful, all-knowing, and wholly good, why is there so much suffering?
The Problem of Inductionconcept · logic · EnlightenmentEvery inference from what we have observed to what we have not observed rests on an assumption that cannot itself be justified without circularity.
The Problem of Universalsconcept · philosophy · AncientDo properties and kinds like redness or humanity exist as real things shared by many objects, or are they only names we give to resemblances among particulars?
Problem-Solving Heuristicsconcept · psychology · Late 20th centuryThe rules of thumb people use to search for a solution when no guaranteed procedure exists, and the fixed habits of mind that can make those shortcuts fail.
Procedural Justiceconcept · philosophy · Late 20th centuryJustice may lie not in the outcome but in the fairness of the process that produced it.
Product-Market Fitconcept · economics · ContemporaryThe point at which a product satisfies a strong market so well that demand pulls it out of the company faster than the company can supply it.
Prototypes as questionsmodel · engineering · Cross-eraA prototype is not a small version of the final product, it is the cheapest object you can build that answers one specific uncertain question.
Psychiatric Classificationconcept · psychology · Late 20th centuryThe project of sorting mental suffering into named, criteria-based categories, useful enough to build a field on and contested at every level, from whether clinicians agree on it to whether the categories are real.
The Psychology of Attractionconcept · psychology · Late 20th centuryAttraction is not random: proximity, similarity, and physical attractiveness are the three predictors that decades of research keep confirming.
Public Goodsconcept · economics · Late 20th centuryA public good is one that everyone can use without reducing what remains for others, and from which no one can practicably be excluded.
The Pythagorean relationshipconcept · mathematics · Cross-eraIn any right triangle, the area of the square on the hypotenuse always equals the combined area of the squares on the other two sides, a fact provable by rearranging area rather than by measuring it.
Quantity before numberconcept · mathematics · Cross-eraBefore any numeral exists, a quantity is just how much of something there is, and you can compare, match, and preserve it without ever naming a number.
Rationalism and Empiricismconcept · philosophy · Early modernThe early-modern quarrel over where knowledge comes from: reason and innate structure, or the senses and experience.
Ratios, proportions, and similarityconcept · mathematics · Cross-eraA ratio is a relationship between two quantities, of the form how many times one contains the other, that can stay exactly fixed while the quantities themselves grow or shrink together.
Reconstructive Memoryexperiment · psychology · Late 20th centuryMemory rebuilds an event each time it is recalled, and information encountered afterward can be woven into that rebuild until people sincerely remember what never happened.
Redundancymodel · logic · Cross-eraCarry more capacity than the average day requires, because failure comes on the days that are not average.
Reference frames and relative descriptionconcept · physics · Cross-eraMotion has no meaning stated in isolation; every description of position, velocity, or rest is a description relative to a chosen frame of reference, and changing the frame changes the description without changing what physically happens.
Regression to the Meanmodel · logic · 19th centuryWhen you measure something extreme, the next measurement tends to be less extreme, because luck does not repeat, and mistaking that drift for cause is how we fool ourselves.
The relation between force, mass, and accelerationconcept · physics · Early modernNet force, mass, and acceleration are locked together by one relation, net force equals mass times acceleration, and it can be reconstructed, not just memorized, from how force and mass were separately defined.
Reliability and Validityconcept · psychology · Early 20th centuryThe two axes of measurement quality: reliability is whether a measure is consistent, validity is whether it measures the thing it claims to.
René Descartesthinker · philosophy · Early modernThe philosopher who tried to rebuild all knowledge from a single unshakable point, and found it in the one thing doubt could not touch: the doubter.
The Replication Crisisconcept · psychology · ContemporaryThe discovery that a large share of landmark psychology findings could not be reproduced, traced to small samples, flexible statistics, and a publishing system that rewards novelty over truth.
Representative Democracyideology · politics · Late 20th centuryGovernment in which the people rule not by voting on policies themselves but by electing representatives who govern in their name and answer to them at the next election.
The Republictext · philosophy · ClassicalPlato's dialogue that asks whether justice pays even when no one is watching, and answers by building a just city in words to see justice in the soul.
Republicanismideology · politics · AncientA politics in which rule is a public matter, held in common and without a master, aimed at the common good and secured by the freedom of citizens to live free from domination.
The Repugnant Conclusionexperiment · ethics · Late 20th centuryIf total welfare is what matters, an enormous population of lives barely worth living beats a small population of wonderful ones, and Parfit found every escape route blocked.
Requirements and success criteriamodel · engineering · Cross-eraA requirement is a single, checkable statement of what a design must achieve, written so that WHAT is fixed while HOW is left entirely open.
Research Ethics and Its Failuresconcept · psychology · Early 20th centuryThe rules that now govern experiments on people were written in reaction to specific abuses, and each principle is a scar left by a study that went wrong.
Responsibility to Protectconcept · politics · ContemporaryThe doctrine that sovereignty carries a duty to protect a population from atrocity, and that when a state fails, that duty passes to the world.
Rightsconcept · philosophy · ClassicalA right is a justified claim that others, including the state, are bound to respect.
Robert Nozickthinker · politics · Late 20th centuryThe philosopher who argued that only a minimal state can be just, because individuals have rights so strong that no pattern of distribution may override them.
Rotational inertiaconcept · physics · Cross-eraRotational inertia measures how hard it is to change a body's spin, and it depends not just on how much mass there is, but on how far that mass sits from the axis of rotation.
The Rubber Band Modelmodel · psychology · ContemporaryA hard decision about whether to leave is a rubber band stretched between the bonds that hold you and the pull of something new; name both before you decide.
The Rubicon Modelmodel · psychology · Late 20th centuryAction runs through four phases split by a single point of no return, the moment you decide, and each side of that line demands a different frame of mind.
The Rule of Lawconcept · politics · ClassicalThe principle that public power must be exercised through general, publicly known rules rather than arbitrary personal command.
Rule Utilitarianismconcept · ethics · Late 20th centuryDo not ask what this act would do; ask what would happen if everyone followed the rule it falls under, and follow the rules whose general acceptance would produce the most good.
Safety factors and uncertaintyconcept · engineering · Cross-eraA safety factor is the ratio of the load a part is designed to survive to the load it is actually expected to carry, sized deliberately to absorb the variability in materials, manufacturing, loading, and analysis that no calculation can fully eliminate.
The Sapir-Whorf Hypothesisconcept · psychology · Early 20th centuryThe claim that the language you speak shapes how you think, strongly supported in its modest form and rejected in its extreme one.
Schelling's Segregation Modelmodel · economics · Late 20th centuryMild individual preferences for similar neighbors produce extreme collective segregation.
Second-Order Thinkingmodel · logic · Late 20th centuryAsk not just what an action does, but what happens after that: the consequences of the consequences.
Second-Wave Feminismideology · politics · Late 20th centuryThe claim that women's inequality lives not only in law but in the everyday structures of home, work, sex, and self: the personal is political.
Securitizationconcept · politics · Late 20th centurySecurity is not a fact about the world but a move in politics: to call something an existential threat is to claim a license for emergency action.
The Security Dilemmaconcept · politics · Late 20th centuryUnder anarchy, the steps a state takes to make itself safer can make others less safe, so they arm in reply and everyone ends up more exposed, even when no one intended harm.
Selective Attentionconcept · psychology · Late 20th centuryThe mind can attend to only a fraction of what reaches the senses, and how it chooses, by a filter at the ears or by turning down what it ignores, is the central puzzle of early attention research.
Self-Determinationconcept · politics · EnlightenmentThe claim that a people has the right to govern itself, free from external domination.
Self-Determination Theorymodel · psychology · Late 20th centuryThe theory that people are healthiest and most durably motivated when three basic needs, autonomy, competence, and relatedness, are met, and that external rewards can crowd out the internal motivation they aim to boost.
Self-Efficacyconcept · psychology · Late 20th centuryThe belief that you can carry out the specific actions a situation demands, which shapes whether you attempt it, how hard you try, and how long you persist.
Sense and Referenceconcept · philosophy · 19th centuryAn expression carries two things at once: what it points at in the world (its reference) and the way it presents that thing (its sense).
The Separation of Powersconcept · politics · EnlightenmentLiberty is safeguarded by dividing the powers of government into distinct branches so that no single person or body holds them all.
The Seven Perspectives of Psychologymodel · psychology · 19th centuryPsychology explains the same behavior at seven different levels, from brain chemistry to culture, and the levels are complementary answers to different questions rather than rival theories competing for one.
The Ship of Theseusexperiment · logic · AncientIf every plank of a ship is replaced one by one, is it still the same ship? A puzzle about what makes a thing the same thing over time.
Sigmund Freudthinker · psychology · 19th centuryThe Viennese neurologist who invented psychoanalysis and gave the modern world its working vocabulary for the unconscious mind, on evidence that most philosophers of science now judge too weak to support it.
Signal Detection Theorymodel · psychology · Late 20th centuryA framework separating how well you can tell signal from noise from how willing you are to say yes when unsure.
Signalingmodel · economics · Late 20th centuryWhen quality is hidden, a signal is credible only if it costs the impostor more than it costs the genuine article.
Sine, cosine, and tangent as geometric ratiosconcept · mathematics · Cross-eraSine, cosine, and tangent are ratios between the sides of a right triangle that depend only on one angle, never on the triangle's size, because similar triangles always share the same side ratios.
Sleep and Dreamingconcept · psychology · Late 20th centurySleep is a structured, actively regulated brain state cycling through distinct stages, and dreams are the mind's activity within it, explained by rival theories no one has settled.
Social Capitalconcept · politics · Late 20th centuryThe stock of trust, reciprocity, and social connection that lets people cooperate, and that Putnam argued was quietly draining out of American life.
The Social Contractconcept · politics · Early modernThe idea that political authority is legitimate only because the governed would, or do, agree to it.
Social Democracyideology · politics · Late 20th centuryReform capitalism from within: keep the market, but harness it with a strong welfare state, universal services, and organized bargaining, all pursued by the ballot rather than the barricade.
Social Entrepreneurship and the Triple Bottom Lineconcept · economics · Late 20th centuryRun an enterprise to solve a social problem, and judge it on three accounts, people, planet, and profit, not on financial return alone.
Social Facilitation and Social Loafingconcept · psychology · Late 20th centuryOther people can make you try harder or slacker: their mere presence sharpens easy tasks and spoils hard ones, while being pooled anonymously into a group makes everyone quietly ease off.
Social Identity Theoryconcept · psychology · Late 20th centuryThe theory that sorting people into groups, even trivial and arbitrary ones, is enough by itself to make them favor their own group.
Social Liberalismideology · politics · 19th centuryLiberalism that keeps liberty as its end but accepts an active state to secure the material conditions that make liberty real.
Socialismideology · politics · 19th centuryThe conviction that the major means of production should be owned and controlled in common, so that the economy serves human need rather than private profit.
Socratesthinker · philosophy · ClassicalThe Athenian who wrote nothing, questioned everything, and made the examined life the business of philosophy, then died rather than stop.
Soft Powerconcept · politics · Late 20th centurySoft power is getting others to want what you want, through the pull of your culture, values, and legitimacy, rather than through force or payment.
Sovereigntyconcept · politics · Early modernSovereignty is the claim that some authority within a territory has the final and supreme power to make, enforce, and adjudicate law, answerable to no higher earthly power.
The Spacing Effectmodel · psychology · 19th centuryThe same amount of study spread out over time produces far more durable learning than the same study packed into one session.
Speed and velocityconcept · physics · Cross-eraSpeed is how fast the distance odometer is ticking, a plain number with no direction, while velocity is the rate of change of displacement, a directed quantity that can be zero, positive, or negative even while speed stays constant.
Split-Brain Researchexperiment · psychology · Late 20th centuryWhen the cable joining the two hemispheres is cut, each half of the brain can perceive, decide, and act on its own, and the talking half calmly invents reasons for what the mute half just did.
Springs, dampers, and suspensionconcept · engineering · Cross-eraA suspension pairs a spring, which stores energy and pushes back proportional to displacement, with a damper, which dissipates energy proportional to speed, so a vehicle can absorb a bump and then stop moving rather than bounce indefinitely.
The Stages of Griefconcept · psychology · Late 20th centuryFive stages, denial, anger, bargaining, depression, acceptance, that Elisabeth Kübler-Ross described in the dying and the public later mistook for a map of how everyone grieves.
Stalinismideology · politics · Early 20th centuryThe system Joseph Stalin built in the Soviet Union: rapid state-driven industrialization and forced collectivization, enforced by mass terror, a vast prison-camp economy, and a cult of the leader.
The Stanford Prison Experimentexperiment · psychology · Late 20th centuryA six-day Stanford simulation in which student guards turned abusive, taken for decades as proof of situational power until 2018 archives showed the abuse was coached, not spontaneous.
The State of Natureconcept · philosophy · Early modernA hypothetical condition before government existed, used to ask what justifies political authority at all.
Static equilibriumconcept · physics · Cross-eraA rigid body genuinely at rest must satisfy two independent conditions at once: zero net force, so it does not start moving, and zero net torque, so it does not start turning.
Static frictionconcept · physics · Cross-eraStatic friction is not a fixed force but a responsive one, matching whatever is needed to keep two touching surfaces from sliding, up to a maximum set by the normal force and a material coefficient.
Statistical Powerconcept · psychology · Early 20th centuryThe probability that a study will detect a real effect if one exists, and the quiet reason so many studies fail to.
Statistical Significanceconcept · psychology · Early 20th centuryThe p-value measures how surprising your data would be if nothing were going on, and almost everyone reads it as something else.
Sternberg's Triangular Theory of Lovemodel · psychology · Late 20th centuryLove is built from three components (intimacy, passion, and commitment) whose presence or absence yields eight recognizable kinds of love.
Stoicismtradition · philosophy · ClassicalA Greek and Roman philosophy holding that the good life is a life of virtue lived in accordance with reason and nature, and that our peace depends on distinguishing what is up to us from what is not.
Structural Realismconcept · politics · Late 20th centuryState behavior is driven by the structure of an anarchic system and the distribution of power within it, not by human nature or the kind of regime a state has.
Structuralism versus Functionalismconcept · psychology · 19th centuryPsychology's founding argument: dissect the mind into its raw elements, or ask what mental processes do for the organism, and the second question proved the more useful science.
Supererogationconcept · ethics · Late 20th centuryThe morally good beyond the call of duty: acts that are praiseworthy to do but not blameworthy to omit.
Supply and Demandmodel · economics · EnlightenmentPrice is the point at which the quantity people want to buy equals the quantity people want to sell.
Surplus Valueconcept · economics · 19th centuryWorkers produce more value than they are paid; the difference, surplus value, is the source of capitalist profit.
The Swiss Cheese Modelmodel · logic · Late 20th centuryAccidents happen not because one defense fails but because the holes in many defenses momentarily line up to let a hazard through.
SWOT Analysismodel · economics · Late 20th centuryA planning grid that sorts what you know about a decision into four boxes: internal Strengths and Weaknesses, external Opportunities and Threats.
Symmetry and invarianceconcept · mathematics · Cross-eraA symmetry is a transformation that leaves an object or a situation looking exactly the same, and whatever stays unchanged under that transformation is called invariant, a fact that can be used to shortcut a calculation rather than merely decorate its answer.
Syndicalismideology · politics · Early 20th centuryThe doctrine that workers should abolish the state and capitalism through their own industrial unions, using direct action and the general strike rather than the ballot or the party.
System boundaries and interfacesmodel · engineering · Cross-eraA system boundary is the deliberately chosen line separating what you are designing from everything else, and an interface is every point where matter, energy, or information crosses that line.
Technocracyideology · politics · Late 20th centuryRule, or at least decisive rule-setting, handed to experts and technical criteria rather than to elected generalists or popular will.
Tension in strings and cablesconcept · physics · ClassicalTension is the pulling force transmitted along a flexible connector such as a string or cable, uniform along an ideal massless, frictionless length and solved for by applying Newton's second law to each body it connects.
Tension, compression, bending, torsion, and shearconcept · engineering · Cross-eraEvery internal force a member carries can be sorted into five basic types, tension, compression, bending, torsion, and shear, and telling them apart is what lets you predict how and where a part will actually fail.
Theocracyideology · politics · AncientRule in the name of God, where religious law is the law of the state and clergy either hold power or certify who may.
Theories of Aggressionconcept · psychology · Early 20th centuryThree accounts of why people harm one another, aversive circumstance, learned imitation, and evolved biology, that turn out to describe different stages of one process rather than rival answers.
Theories of Color Visionconcept · psychology · 19th centuryTwo rival accounts of how the eye sees color, three receptor types versus opposed color pairs, that modern neuroscience shows are both right, at different stages of the visual pathway.
Theories of Emotionconcept · psychology · Early 20th centuryFour rival answers to a single question: does feeling come from the body, the brain, the label you give your arousal, or the meaning you assign to the situation.
Theories of Intelligenceconcept · psychology · Early 20th centuryFrom Spearman's single general factor to Gardner's many independent intelligences, the century-long argument over whether mental ability is one thing or many, and what IQ tests actually capture.
Theories of Truthconcept · philosophy · AncientRival accounts of what makes a proposition true: matching the world, fitting a system, working in practice, or nothing substantive at all.
Theory of Mindconcept · psychology · Late 20th centuryThe capacity to attribute mental states, beliefs, desires, intentions, to others and to grasp that theirs can differ from your own and from reality.
Third-Wave Feminismideology · politics · Late 20th centuryThe feminism that abandoned the universal 'woman' for difference, intersectionality, and individual agency.
Thomas Hobbesthinker · philosophy · Early modernHe argued that without a sovereign power holding absolute authority, human life would collapse into a war of all against all.
Thomas Malthusthinker · economics · EnlightenmentThe clergyman-economist who argued that population, left unchecked, will always outrun the food supply, and that this tension is the permanent condition of human societies.
Thomas Painethinker · politics · EnlightenmentThe corsetmaker turned revolutionary pamphleteer who told the American colonies to break with monarchy and told the world that government exists only to secure natural rights.
Thorstein Veblenthinker · economics · 19th centuryThe sardonic outsider who revealed that economic life is driven less by rational calculation than by status, habit, and the predatory instincts of a leisure class.
The Thucydides Trapconcept · politics · ContemporaryThe idea that when a rising power threatens to displace a ruling one, the resulting structural stress often ends in war.
The Ticking Time Bombexperiment · ethics · Late 20th centuryA hypothetical bomb, a captured terrorist, and a clock: the scenario built to make torture look not only permissible but obligatory.
Torque and rotational effectconcept · physics · Cross-eraTorque is the turning effect of a force, set by how much force you apply, how far from the pivot, and at what angle.
Totalitarianismideology · politics · Late 20th centuryThe attempt to seize total control of society, ideology, and the inner life through one party, terror, propaganda, and an official truth.
Trade-off spaces and objective functionsmodel · engineering · Cross-eraA trade-off space is the set of designs where improving one property necessarily worsens another, and an objective function is the explicit rule you use to decide which point in that space you actually want.
Traditionalist Conservatismideology · politics · EnlightenmentSociety is an inheritance to be tended, not a machine to be redesigned: reform slowly, distrust abstract blueprints, and keep faith with the dead and the unborn.
The Tragedy of the Commonsconcept · economics · Late 20th centuryA shared resource, left unregulated, tends toward ruin because each user gains by taking more even as collective overuse destroys it.
Transnational Advocacy Networksmodel · politics · Late 20th centuryWhen a government blocks its own citizens, they route around it: appealing to allies abroad who pressure the state from outside, so the demand comes back home like a boomerang.
The Transtheoretical Modelmodel · psychology · ContemporaryA map of behavior change as a sequence of five stages, from not intending to change to sustaining a change, with different help suited to each.
Triangles and rigidityconcept · mathematics · Cross-eraA triangle with three fixed side lengths has exactly one possible shape, which is why triangulated frames resist deformation that four sided frames cannot.
The Trolley Problemexperiment · ethics · Late 20th centuryA pair of near-identical dilemmas designed so that our intuitions diverge, forcing us to say what actually makes killing wrong.
Trotskyismideology · politics · Late 20th centuryThe revolutionary Marxism of Leon Trotsky: workers' revolution must be permanent and international, and the Stalinist USSR was a workers' state betrayed by its bureaucracy.
Trusses, frames, and triangulationconcept · engineering · Cross-eraA truss carries load through a rigid arrangement of triangles, so that nearly every member works only in tension or compression, letting a structure be strong while using far less material than a solid beam of the same span.
Two-dimensional motion and projectilesconcept · physics · Cross-eraMotion in two dimensions, including a thrown or launched projectile's curved path, can be built exactly by treating the horizontal and vertical motions as two independent one-dimensional motions happening at the same time, sharing only the clock.
Two-Level Gamesmodel · politics · Late 20th centuryInternational negotiation is a game played at two tables at once: bargaining with foreign counterparts while selling any deal back home.
Unit Economicsconcept · economics · Late 20th centuryJudge a business one customer at a time: if it loses money on each customer, growth makes it worse, not better.
Units and dimensional reasoningconcept · mathematics · Cross-eraA measured number is incomplete without its unit, because the unit names the standard the number is a multiple of, and mismatched units make an equation meaningless regardless of whether the digits agree.
Units, standards, and calibrationconcept · physics · Cross-eraA unit is an agreed, reproducible reference amount, and calibration is the act of comparing an instrument against that reference so its readings can be trusted by anyone, not just its owner.
Universal gravitationconcept · physics · Early modernEvery two masses in the universe pull on each other with a force that grows with both masses and falls off as the square of the distance between them, the same law that governs a falling apple and a moon in orbit.
The Universality of Facial Expressionsconcept · psychology · Late 20th centuryPaul Ekman's claim that a handful of basic emotions wear the same face in every culture, and the unsettled scientific fight over whether the evidence actually supports it.
The Upanishadstext · religion · AncientThe concluding portion of the Vedas, where Indian thought turns from ritual to the inner self and asks what, if anything, is ultimately real.
Utilitarianismconcept · ethics · EnlightenmentThe right action is the one that produces the greatest happiness for the greatest number, counting everyone's happiness equally.
Variables and unknownsconcept · mathematics · Cross-eraA variable is a symbol paired with a domain of allowed values, standing for an unknown, a varying quantity, or a placeholder for generality depending on how it is used.
Vectors as directed quantitiesconcept · mathematics · 19th centuryA vector is a quantity that has both a magnitude and a direction, and that combines with others by placing them tip to tail rather than by ordinary addition of numbers.
The Veil of Ignoranceexperiment · ethics · Late 20th centuryA device for finding just rules: choose the principles of your society without knowing who in it you will be.
The Venture Capital Power Lawconcept · economics · ContemporaryVenture returns are so skewed that a fund's single best investment usually returns more than all the others combined, so the winning strategy is to maximize exposure to extreme upside, not to avoid loss.
Veto Playersmodel · politics · Late 20th centuryPolicy change requires the agreement of every actor whose consent is needed to pass it; the more of them there are, the more ideologically distant, and the less internally unified, the harder change becomes.
Via Negativamodel · logic · Cross-eraImprove by subtraction: the surest gains often come from removing the harmful rather than adding the good.
Virtue Ethicsconcept · ethics · ClassicalThe ethics not of what you should do but of who you should be: good action flows from good character, and the central question is what kind of person to become.
Vladimir Leninthinker · politics · 19th centuryThe theorist and revolutionary who remade Marxism into a doctrine of disciplined party seizure of power, then tested it on the largest country on earth.
War Made the Statemodel · politics · Late 20th centuryModern states were built not by contract or design but as the by-product of rulers scrambling to fund war: to fight, they had to tax, and to tax they had to build bureaucracy.
Weber's Lawconcept · psychology · 19th centuryThe just-noticeable difference between two stimuli is a roughly constant fraction of their magnitude, so we perceive changes by ratio, not by absolute amount.
Weight and gravitational interactionconcept · physics · Early modernWeight is not a property an object owns; it is the specific gravitational force pulling on it right now, which is why the same object weighs differently on the Moon, in an accelerating elevator, and in orbit, while its mass never changes at all.
The WEIRD-Sample Critiqueconcept · psychology · ContemporaryThe finding that most published psychology describes people who are Western, Educated, Industrialized, Rich, and Democratic, a narrow and often unrepresentative slice of the species mistaken for a universal baseline.
What Is It Like to Be a Bat?experiment · philosophy · Late 20th centuryThere is something it is like to be a conscious creature, and Thomas Nagel argues that this inside view is exactly what a purely physical description of the brain can never capture.
Wheels, rolling resistance, and tractionconcept · engineering · Cross-eraA wheel trades most sliding friction for a much smaller rolling resistance, but the same contact patch must still provide, through static friction, all the traction a vehicle needs to accelerate, brake, and turn.
Wilhelm Wundtthinker · psychology · 19th centuryThe founder of psychology as an experimental science, whose disciplined method and two-part program were later flattened into a caricature by the student who claimed to be his heir.
Wisdom and Successful Agingconcept · psychology · ContemporaryWhat psychology actually finds when it measures wisdom and studies who ages well: some abilities decline, others hold or grow, and wisdom is rarer and less age-bound than the folk picture assumes.
Work as force through distanceconcept · physics · 19th centuryWork is the transfer of mechanical energy by a force, measured as the component of that force along an object's displacement times the distance moved, and it is zero whenever force and motion are perpendicular.
The Working Memory Modelmodel · psychology · Late 20th centuryShort-term memory is not one holding pen but a small team: an executive that directs attention, a sound-based loop, a visual sketchpad, and a buffer that binds them into experience.
World-Systems Theorymodel · politics · Late 20th centuryThere is one capitalist world-economy, not many national economies climbing the same ladder, and it is built as a single division of labor that enriches a core by draining a periphery.
The Yerkes-Dodson Lawmodel · psychology · Early 20th centuryPerformance improves as arousal rises but only up to a peak, and that peak sits lower for hard tasks than for easy ones.
Zero as number, placeholder, and boundaryconcept · mathematics · Cross-eraZero is not one idea but four: a placeholder that fixes position, a count of an empty collection, the boundary between positive and negative, and the number that leaves every quantity unchanged under addition.