Short definitions of the key terms used in this guide. Each entry links to the section where the concept is explained in full. Terms are alphabetized ignoring articles and punctuation.
A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · R · S · T · U · V · W
A
A posteriori. Known (or knowable) only through experience of the world. “Water boils at 100 °C at sea level.” → Ch. 1
A priori. Knowable independently of experience, beyond the experience needed to grasp the concepts involved. “7 + 5 = 12.” → Ch. 1, Ch. 7
Abduction. Inference to a hypothesis because it would best explain the evidence; also called inference to the best explanation. → Ch. 3, Ch. 10
Acceptance. Adopting a proposition as a premise for reasoning or action in some context, whether or not one believes it. → Ch. 1
Access internalism. The view that whatever justifies a belief must be accessible to the believer by reflection. → Ch. 6
Ad hoc hypothesis. A modification introduced solely to save a theory from refutation, without independent testable consequences. → Ch. 10
Ad hominem. Attacking the person who makes an argument rather than the argument itself. → Ch. 15
Affirming the consequent. The invalid form “If P then Q; Q; therefore P.” → Ch. 3
Agrippa’s trilemma. The argument that every attempt at justification ends in an infinite regress, a circle, or an arbitrary assumption. Also called the Münchhausen trilemma. → Ch. 6
Ambiguity. Having two or more distinct meanings (lexical, syntactic, scope, collective/distributive). Distinct from vagueness. → Ch. 4
Analytic. True in virtue of meaning alone (or, for Kant, the predicate is contained in the subject concept). “All bachelors are unmarried.” → Ch. 1
Anchoring. The tendency for estimates to be pulled toward an initial value, even an irrelevant one. → Ch. 14
Anti-realism. The view that truth in some domain is tied to what we could verify or recognize, or (in philosophy of science) that we need not believe theories are true about unobservables. → Ch. 11, Ch. 10
Apt belief. Sosa’s term for a belief that is true because it manifests the believer’s competence. → Ch. 5
Argument. A set of statements in which some (premises) are offered as reasons for another (conclusion). → Ch. 3
Argument from ignorance. Concluding that something is true because it hasn’t been proven false, or vice versa. → Ch. 15
Argument mapping. Diagramming the structure of reasoning: premises, conclusions, objections, and rebuttals. → Ch. 3
Availability heuristic. Judging frequency or probability by how easily examples come to mind. → Ch. 14
B
Base rate. The prior frequency of something in a population; ignoring it produces base rate neglect. → Ch. 9
Basic belief. In foundationalism, a belief justified non-inferentially, not on the basis of other beliefs. → Ch. 6
Basing relation. The relation between a belief and the reasons for which it is actually held. → Ch. 6
Bayes’ theorem. The rule for updating the probability of a hypothesis given evidence: posterior odds = prior odds × likelihood ratio. → Ch. 9
Bayesian epistemology. The theory that rational degrees of belief obey the probability axioms and are updated by conditionalization. → Ch. 9
Begging the question. Arguing in a way that assumes the conclusion; circular reasoning. → Ch. 15
Belief. The attitude of taking a proposition to be true. → Ch. 1
Brain in a vat. A skeptical scenario in which a disembodied brain receives computer-generated experiences indistinguishable from normal life. → Ch. 8
Brier score. A measure of the accuracy of probabilistic forecasts: the mean squared difference between forecast probabilities and outcomes. → Ch. 9
Bullshit (technical sense). Speech produced with indifference to whether it is true (Frankfurt). → Ch. 11
Burden of proof. The obligation to support a claim; the party who bears it loses if no decisive argument is given. → Ch. 16
C
Calibration. The match between one’s confidence and one’s accuracy: being right about 70% of the time when 70% confident. → Ch. 9
Charity, principle of. Interpreting others’ statements in the most reasonable way available. → Ch. 3
Closure principle. If you know p and know that p entails q, you know (or are in a position to know) q. → Ch. 8
Cogency. The property of an inductive argument that is strong and has true (or acceptable) premises. → Ch. 3
Coherence theory of truth. The view that a proposition is true if it coheres with a system of propositions. → Ch. 11
Coherentism. The view that beliefs are justified by their coherence with a whole system of beliefs, not by basic foundations. → Ch. 6
Conciliationism. The view that on learning that an epistemic peer disagrees, one should move substantially toward their view. → Ch. 12
Conditionalization. Updating credences by setting the new probability of H equal to the old conditional probability of H given the evidence learned. → Ch. 9
Confirmation bias. The tendency to seek, interpret, and recall information in ways that confirm existing beliefs. → Ch. 14
Confounding. A third factor that causes both of two correlated variables, creating a misleading association. → Ch. 10
Conjunction fallacy. Judging a conjunction (A and B) more probable than one of its parts. → Ch. 9
Consensus (scientific). Agreement among most experts in a field, usually resulting from convergent lines of evidence. → Ch. 10
Contextualism (epistemic). The view that the truth of “S knows that p” depends on the standards in play in the attributor’s context. → Ch. 8
Contingent. True but could have been false. → Ch. 1
Continuum fallacy. Inferring that there is no real difference between X and Y because there is no sharp line between them. → Ch. 4
Correspondence theory of truth. The view that a proposition is true if it corresponds to the facts. → Ch. 11
Credence. A degree of belief, represented as a number between 0 and 1. → Ch. 1
Critical rationalism. Popper’s view that beliefs cannot be justified, only criticized, and that rationality consists in openness to criticism. → Ch. 6
Crux. A point on which a disagreement depends; settling it would change someone’s mind. → Ch. 16
Curse of knowledge. See Epistemic egocentrism.
Cynical Theory. Nagel’s name for the view that “knowledge” is merely a label for the opinions of the powerful. → Ch. 11
D
De re / de dicto. The distinction between a claim about a thing (de re) and a claim about a proposition or description (de dicto). → Ch. 3
Deduction. Reasoning in which the premises are meant to guarantee the conclusion. → Ch. 3
Deep disagreement. Disagreement over what counts as evidence or authority, so that the parties share no common ground for argument. → Ch. 12
Defeater. A consideration that removes or reduces a belief’s justification. Rebutting defeaters are reasons against the conclusion; undercutting defeaters are reasons against the support. → Ch. 6
Deflationism. The view that there is nothing substantial to say about the nature of truth beyond the T-schema. → Ch. 11
Demarcation problem. The problem of distinguishing science from pseudoscience and non-science. → Ch. 10
Disjunctivism. The view that veridical perception and hallucination are fundamentally different kinds of mental state. → Ch. 7
Dogmatism (Pryor’s technical sense). The view that perceptual experience gives immediate prima facie justification, without prior justification that perception is reliable. → Ch. 8
Doxastic. Pertaining to belief (from Greek doxa). The doxastic attitudes are belief, disbelief, and suspension of judgment. → Ch. 1
Doxastic voluntarism. The view that we can choose what to believe. → Ch. 6
Dual-process theory. The view that cognition involves fast, intuitive (System 1) and slow, deliberate (System 2) processes. → Ch. 14
Duhem–Quine thesis. Hypotheses are tested only in conjunction with auxiliary assumptions, so a failed prediction does not show which hypothesis is false. → Ch. 10
Dunning–Kruger effect. The finding that low performers overestimate their competence; its interpretation is disputed. → Ch. 14
Dutch book. A set of bets that guarantees a loss; credences that violate probability axioms are vulnerable to one. → Ch. 9
E
E = K. Williamson’s thesis that one’s evidence is exactly one’s knowledge. → Ch. 5
Echo chamber. A social structure in which outside voices are actively discredited. Contrast with an epistemic bubble. → Ch. 12
Ecological rationality. The view that heuristics are rational when adapted to the structure of the environment. → Ch. 14
Empiricism. The view that knowledge derives chiefly or entirely from experience. → Ch. 2
Epistemic bubble. An information network from which other voices are omitted, but not discredited. → Ch. 12
Epistemic egocentrism. The difficulty of setting aside one’s own knowledge when judging someone who knows less; also called the curse of knowledge. → Ch. 14
Epistemic injustice. A wrong done to someone in their capacity as a knower, in testimonial or hermeneutical forms. → Ch. 12
Epistemic luck. Luck in how a belief comes to be true. Veritic luck is incompatible with knowledge. → Ch. 5
Epistemic vigilance. The largely unconscious monitoring of informants for competence and honesty. → Ch. 7
Epistemology. The study of knowledge, belief, evidence, justification, and rationality. → Ch. 1
Equivocation. Using a word in different senses within one argument. → Ch. 15
Essentially contested concept. A concept whose proper application is inherently disputed, such as democracy or justice (Gallie). → Ch. 4
Evidence. What makes a proposition more or less likely to be true; E is evidence for H if E is more probable given H than given not-H. → Ch. 1, Ch. 9
Evidentialism. The view that a belief is justified if and only if it fits the believer’s evidence. → Ch. 6
Externalism. The view that justification or knowledge can depend on factors outside the subject’s awareness, such as reliability. → Ch. 6
F
Factive. A verb or state that entails the truth of its complement: “knows that p” entails p. → Ch. 5
Fallacy. A common pattern of bad reasoning that seems persuasive. → Ch. 15
Fallibilism. The view that we can know (or justifiably believe) something even though our grounds don’t guarantee its truth. → Ch. 6
Falsifiability. Popper’s criterion: a theory is scientific if some possible observation could refute it. → Ch. 10
False-belief task. A test of whether someone can attribute a mistaken belief to another; children typically pass explicit versions around age four or five. → Ch. 14
Family resemblance. Wittgenstein’s term for a concept whose instances share overlapping similarities but no single common feature. → Ch. 4
Foundationalism. The view that justified beliefs rest ultimately on basic beliefs that are justified non-inferentially. → Ch. 6
Foundherentism. Haack’s hybrid of foundationalism and coherentism, modeled on a crossword puzzle. → Ch. 6
Framing effect. Different choices produced by logically equivalent descriptions of the same options. → Ch. 14
G
Gettier case. A case of justified true belief that is not knowledge because the belief is true by luck. → Ch. 5
Groupthink. A cohesive group’s tendency to suppress dissent and fail to consider alternatives. → Ch. 12
Grue. Goodman’s predicate (green if examined before t, blue otherwise) illustrating the new riddle of induction. → Ch. 9
H
Heuristic. A mental shortcut for judgment or decision. → Ch. 14
Higher-order evidence. Evidence about one’s own capacity to evaluate evidence. → Ch. 12
Hinge epistemology. The view, from Wittgenstein’s On Certainty, that some propositions are presupposed by all inquiry and are not themselves objects of doubt or evidence. → Ch. 8
Hume’s fork. The division of all knowledge into relations of ideas and matters of fact. → Ch. 2
I
Illusion of explanatory depth. Believing one understands how things work much better than one does. → Ch. 14
Illusory truth effect. Repeated statements are judged more likely to be true. → Ch. 14
Incommensurability. Kuhn’s claim that rival paradigms lack a common measure for comparison. → Ch. 10
Induction. Reasoning from observed cases to general conclusions or to unobserved cases. → Ch. 3, Ch. 9
Inductive risk. The risk of error in accepting or rejecting a hypothesis; its consequences affect how much evidence should be required. → Ch. 10
Inference to the best explanation. See Abduction. → Ch. 10
Infinitism. The view that justification comes from an infinite, non-repeating chain of available reasons. → Ch. 6
Information cascade. A situation in which people follow others’ choices and ignore their own information, so that errors can lock in. → Ch. 12
Intellectual humility. Being attentive to, and owning, one’s intellectual limitations. → Ch. 13
Intellectual virtue. A character trait or ability that makes someone a good thinker, such as open-mindedness or intellectual courage. → Ch. 13
Interest-relative invariantism. The view that “know” has a fixed meaning but whether a subject knows depends partly on what is at stake for that subject. Also called subject-sensitive invariantism. → Ch. 8
Internalism. The view that justification depends only on factors internal to the subject’s perspective. → Ch. 6
Is–ought gap. Hume’s observation that no normative conclusion follows from purely factual premises. → Ch. 4
J
Justification. The property a belief has when it is held for good reasons or formed appropriately. Propositional justification is having good reasons available; doxastic justification is believing for good reasons. → Ch. 6
Justified true belief (JTB). The classical analysis of knowledge, refuted by Gettier. → Ch. 5
K
KK principle. If one knows p, one knows that one knows p. → Ch. 5
Knowledge by acquaintance. Direct familiarity with a thing, person, or experience (Russell). → Ch. 1
Knowledge by presence (ʿilm ḥuḍūrī). Knowledge in which the object is immediately present to the knower without representation (Suhrawardī). → Ch. 7
Knowledge-first epistemology. Williamson’s view that knowledge is basic and unanalyzable, and explains belief, evidence, and justification. → Ch. 5
Knowledge-how. Practical knowledge of how to do something. → Ch. 1
L
Likelihood ratio. P(E | H) / P(E | not-H): how much more expected the evidence is if the hypothesis is true. → Ch. 9
Lockean thesis. One should believe p if and only if one’s credence in p exceeds a threshold. → Ch. 9
Lottery paradox. High credence that each ticket loses seems to justify believing each will lose, yet one ticket wins. → Ch. 9
M
Masked man fallacy. Substituting co-referring terms inside a belief or knowledge context. → Ch. 4
Meno problem. Why is knowledge more valuable than true belief? → Ch. 5
Mindreading. The capacity to attribute mental states such as knowing, believing, and wanting to others; the source of our intuitions about who knows what. → Ch. 14
Modus ponens. “If P then Q; P; therefore Q.” Modus tollens. “If P then Q; not Q; therefore not P.” → Ch. 3
Mooreanism. Responding to skepticism by holding that we know ordinary truths (such as “I have hands”) better than any skeptical premise. → Ch. 8
Moral encroachment. The view that moral considerations can affect what one is justified in believing. → Ch. 13
Motivated reasoning. Reasoning driven by a desire to reach a particular conclusion. → Ch. 14
Motte and bailey. Advancing a bold claim and retreating to a modest one when challenged. → Ch. 15
Myside bias. Evaluating and generating evidence in a way biased toward one’s own opinions. → Ch. 14
Myth of the Given. Sellars’s critique of the idea that experience provides non-conceptual foundations that can justify beliefs. → Ch. 2
N
Natural frequencies. Presenting probabilities as counts of cases (“9 out of 98”), which improves reasoning. → Ch. 9
Naturalized epistemology. Epistemology continuous with empirical science, especially psychology (Quine). → Ch. 2
Necessary condition. Q is necessary for P if P cannot be true without Q. → Ch. 3
Necessary truth. A truth that could not have been false. → Ch. 1
Neurath’s boat. The image of rebuilding a ship at sea: we must revise beliefs while relying on the rest. → Ch. 2
No-miracles argument. The success of science would be a miracle unless our theories were approximately true. → Ch. 10
Normative. Concerning what ought to be, rather than what is. → Ch. 1
O
Ockham’s razor. Don’t multiply entities beyond necessity; prefer simpler explanations, other things being equal. → Ch. 16
Open-mindedness. Willingness and ability to take alternative views seriously. → Ch. 13
Overconfidence. Overestimation, overplacement, or overprecision in one’s judgments. → Ch. 14
P
P-value. The probability, assuming the null hypothesis and model, of data at least as extreme as those observed. Not the probability that the null hypothesis is true. → Ch. 9
Paradigm. Kuhn’s term for the shared exemplars and framework of a scientific field. → Ch. 10
Peer disagreement. Disagreement with someone who has the same evidence and is equally competent. → Ch. 12
Pessimistic meta-induction. Many successful past theories were false, so current ones probably are too. → Ch. 10
Phenomenal conservatism. If it seems to S that p, S thereby has some justification for p, absent defeaters. → Ch. 6
Pragmatic encroachment. The view that practical stakes affect whether one knows or is justified. → Ch. 13
Pragmatism. The philosophical tradition of Peirce, James, and Dewey, linking meaning and truth to practice and inquiry. → Ch. 2, Ch. 11
Pramāṇa. In Indian philosophy, a means of valid knowledge, such as perception, inference, or testimony. → Ch. 2
Preface paradox. An author rationally believes each claim in her book, and also that at least one is false. → Ch. 9
Prior probability. The probability of a hypothesis before considering new evidence. → Ch. 9
Problem of the criterion. To identify knowledge we need criteria, and to test criteria we need known cases. → Ch. 6
Proposition. What a declarative sentence expresses; the bearer of truth and falsity. → Ch. 1
Propositional knowledge. Knowing that something is the case. → Ch. 1
Prosecutor’s fallacy. Confusing P(evidence | innocence) with P(innocence | evidence). → Ch. 9
R
Rationalism. The view that reason is a major source of knowledge, including substantive a priori knowledge. → Ch. 2, Ch. 7
Reductio ad absurdum. Refuting a claim by showing it leads to a contradiction or absurdity. → Ch. 3
Reductionism (about testimony). The view that testimonial justification reduces to perception, memory, and inference. Global reductionism appeals to the general track record of testimony; local reductionism requires specific reasons to trust each speaker. → Ch. 7
Reflective equilibrium. Adjusting principles and case judgments against each other until they cohere. → Ch. 4
Regress problem. If every justified belief requires justification from another, the chain must end, circle, or go on forever. → Ch. 6
Regression to the mean. Extreme values tend to be followed by values closer to the average. → Ch. 9
Relativism. The view that truth, justification, or values are relative to a framework. → Ch. 11
Relevant alternatives theory. To know p, one must rule out relevant alternatives to p, not all alternatives. → Ch. 8
Reliabilism. The view that knowledge or justification depends on being produced by a reliable process. → Ch. 5, Ch. 6
Replication crisis. The discovery that many published findings fail to replicate. → Ch. 10
Representativeness heuristic. Judging probability by resemblance to a typical case. → Ch. 14
S
Safety. A belief is safe if it could not easily have been false: in nearby worlds where one believes it, it is true. → Ch. 5
Scout mindset. Galef’s term for the motivation to see things as they are, rather than defend one’s beliefs. → Ch. 14
Sensitivity. A belief is sensitive if, were it false, one would not hold it. → Ch. 5
Simpson’s paradox. A trend present in every subgroup can reverse when the groups are combined. → Ch. 9
Skepticism. The view that we know much less than we think, or nothing. → Ch. 8
Slippery slope. Arguing that a first step will lead to a chain of consequences ending in disaster; sometimes fallacious, sometimes not. → Ch. 15
Sorites paradox. The paradox of the heap, arising from vague predicates. → Ch. 4
Soundness. A deductive argument is sound if it is valid and its premises are true. → Ch. 3
Source amnesia. Remembering a fact while forgetting where one learned it; a key test case for internalism. → Ch. 6
Standpoint theory. The view that social position shapes knowledge and that marginalized positions can offer epistemic advantages. → Ch. 12
Stasis theory. The classical method of locating a disagreement at fact, definition, quality, or procedure. → Ch. 16
Steelmanning. Constructing the strongest version of an opposing argument. → Ch. 16
Straw man. Misrepresenting an opponent’s view as weaker, then refuting the misrepresentation. → Ch. 15
Sufficient condition. P is sufficient for Q if P’s truth guarantees Q’s. → Ch. 3
Survivorship bias. Drawing conclusions from the cases that survived a selection process, ignoring those that didn’t. → Ch. 15
Swamping problem. If reliability matters only as a means to truth, it adds no value to a belief that is already true. → Ch. 5
Synthetic. True in virtue of how the world is, not meaning alone. → Ch. 1
Synthetic a priori. Kant’s category of truths that are substantive yet knowable independently of experience. → Ch. 1
T
T-schema. “‘p’ is true if and only if p.” → Ch. 11
Testimonial injustice. Giving a speaker less credibility than deserved because of identity prejudice. → Ch. 12
Testimony. Coming to believe something because someone told you. → Ch. 7
Theory-ladenness. The dependence of observation on the observer’s concepts and theories. → Ch. 10
Toulmin model. A model of argument with claim, data, warrant, backing, qualifier, and rebuttal. → Ch. 3
Tracking. Nozick’s theory that knowledge requires sensitivity and adherence to the truth. → Ch. 5
Truth. On the most common view, correspondence with how things are; theories differ. → Ch. 11
U
Underdetermination. The situation in which the evidence is compatible with rival theories. → Ch. 10
Understanding. Grasping how things fit together and why, often thought more valuable than knowledge. → Ch. 5
Uniqueness thesis. For any body of evidence, exactly one doxastic attitude is rational. Contrast with permissivism. → Ch. 12
V
Vagueness. Having borderline cases where it is unclear whether a term applies. → Ch. 4
Validity. A deductive argument is valid if it is impossible for its premises to be true and its conclusion false. → Ch. 3
Verbal dispute. A disagreement that depends only on different uses of a word. → Ch. 4
Verification principle. The logical positivists’ claim that a statement is meaningful only if analytic or empirically verifiable. → Ch. 2
Virtue epistemology. Epistemology centered on the intellectual abilities and character of knowers. → Ch. 13
W
Warrant. Whatever turns true belief into knowledge (Plantinga’s term). → Ch. 1
Wisdom of crowds. The accuracy of aggregated independent judgments. → Ch. 12