Epistemology — Key Terms and Definitions

General Philosophical Terms

A priori: Knowledge or justification that is independent of experience. A priori truths can be known through reason or thought alone, without needing to observe the world. For example, '2 + 2 = 4' or 'all bachelors are unmarried'.
A posteriori: Knowledge or justification that depends on sense experience of the world. A posteriori truths can only be known by observing the world. For example, 'water boils at 100°C' or 'the cat is on the mat'.
Analytic proposition: A proposition that is true by definition, or true in virtue of the meanings of the words it contains. Denying an analytic truth produces a contradiction. For example, 'all bachelors are unmarried' — the predicate is contained in the subject.
Synthetic proposition: A proposition whose truth or falsity depends on how the world is, not merely on the meanings of the words. Denying a synthetic proposition does not produce a contradiction. For example, 'the cat is on the mat' — this could be true or false depending on the facts.
Necessary truth: A truth that must be true and could not have been false. It is true in all possible worlds. For example, '2 + 2 = 4' is necessarily true — there is no possible situation in which it is false.
Contingent truth: A truth that happens to be true but could have been false. It is true in the actual world but not in all possible worlds. For example, 'it is raining today' — this is true but could easily have been otherwise.
Deductive argument: An argument in which, if the premises are true, the conclusion must be true. The conclusion follows necessarily from the premises. For example: 'All men are mortal; Socrates is a man; therefore Socrates is mortal.'
Inductive argument: An argument in which the premises provide probable but not certain support for the conclusion. The conclusion goes beyond what is contained in the premises. For example: 'Every swan I have seen is white; therefore all swans are white.'
Abductive argument: An inference to the best explanation. The conclusion is accepted not because it follows necessarily from the premises, but because it is the best available explanation of the evidence. For example, inferring that someone was home because the lights are on.
Validity: A property of deductive arguments. An argument is valid if and only if the conclusion follows logically from the premises — it is impossible for the premises to be true and the conclusion false. A valid argument can still have false premises.
Soundness: A property of deductive arguments. An argument is sound if and only if it is valid and all of its premises are true. A sound argument therefore guarantees a true conclusion.
Counter-example: A specific case that disproves a general claim or definition. For example, Gettier cases are counter-examples to the tripartite view because they show cases of justified true belief that are not knowledge.
Necessary condition: A condition that must be met for something to be the case. If the condition is not met, the thing cannot obtain. For example, truth is a necessary condition for knowledge — you cannot know something that is false.
Sufficient condition: A condition (or set of conditions) that, if met, guarantees that something is the case. The tripartite view claims that justified true belief is sufficient for knowledge — if all three conditions are met, you have knowledge.
Hume's Fork: Hume's distinction between two categories of knowledge: relations of ideas (analytic, a priori, necessary) and matters of fact (synthetic, a posteriori, contingent). There is no third category — any claim that falls into neither should be rejected as meaningless.
Occam's Razor: The principle that, all else being equal, the simplest explanation — the one that posits the fewest entities or assumptions — is to be preferred. Used in philosophy to argue against unnecessarily complex theories.
Tautology: A statement that is necessarily true by virtue of its logical form, but tells us nothing about the world. For example, 'it is raining or it is not raining' is true regardless of the weather.
Contradiction: Two claims that cannot both be true at the same time. For example, 'it is raining' and 'it is not raining' cannot both be true simultaneously. A contradictory proposition is necessarily false.

Types of Knowledge

Propositional knowledge: Knowledge that something is the case — 'knowledge-that'. It takes the form 'S knows that P', where P is a proposition. For example, knowing that Paris is the capital of France. This is the type of knowledge that epistemology primarily analyses.
Acquaintance knowledge: Knowledge of something or someone gained through direct experience or familiarity. For example, knowing Paris by having visited it, or knowing a person by having met them. It does not require being able to state any propositions about the thing known.
Ability knowledge: Knowledge of how to do something — 'knowledge-how'. It is practical, procedural knowledge rather than theoretical knowledge. For example, knowing how to ride a bike or knowing how to swim.

The Tripartite View

Tripartite definition of knowledge: The view that knowledge is justified true belief (JTB). S knows that P if and only if: (1) P is true, (2) S believes that P, and (3) S is justified in believing that P. These are claimed to be individually necessary and jointly sufficient conditions for knowledge.
Gettier case: A counter-example to the tripartite view in which a person has a justified true belief that is not knowledge, because the belief is true by luck rather than because of the justification. Gettier cases show that justified true belief is not sufficient for knowledge.
Infallibilism: The view that knowledge requires certainty — a belief counts as knowledge only if it could not possibly be wrong. If there is any chance of error, however small, the belief does not qualify as knowledge.
No false lemmas: A proposed addition to the tripartite view: S knows that P only if S did not infer P from any false belief (a 'false lemma'). This handles original Gettier cases, where the subject reasons from a false intermediate belief, but fails on cases like the barn facade.
Reliabilism: The view that knowledge is true belief produced by a reliable cognitive process — one that generates a high proportion of true beliefs. It replaces the justification condition with a causal requirement, aiming to exclude luck by tying the belief's truth to its origin.
Virtue epistemology: The view that knowledge is true belief that arises from the exercise of intellectual virtues such as careful reasoning, open-mindedness, and perceptual attentiveness. Truth is not an independent condition but flows from the virtuous act of inquiry, dissolving the Gettier problem.

Perception

Direct realism: The view that we perceive mind-independent physical objects directly, without any intermediary. The objects of perception are the physical objects themselves, and they have the properties they appear to have. Also called 'naive realism'.
Indirect realism: The view that we perceive mind-independent objects indirectly, via mind-dependent intermediaries called sense-data. We are directly aware of sense-data, and these represent (or are caused by) external objects. Also called 'representative realism'.
Sense-data: Private, mind-dependent objects of immediate perceptual experience. They have exactly the properties they appear to have — if a sense-datum looks red and round, it is red and round. Used by indirect realists as the intermediary between the perceiver and the external world.
Primary qualities: Properties that belong to objects themselves, independent of any perceiver. Locke's examples include shape, size, motion, and solidity. Primary qualities are objective features of the world that exist whether or not anyone is perceiving them.
Secondary qualities: Properties that are powers in objects to produce certain sensations in perceivers, rather than properties of the objects themselves. Locke's examples include colour, taste, smell, and sound. They depend on the perceiver for their existence.
Berkeley's idealism: The view that no mind-independent material world exists. All that exists are minds and ideas (perceptions). Physical objects are just collections of ideas — to be is to be perceived (esse est percipi). God's continuous perception keeps objects in existence when no human perceives them.
Argument from illusion: An argument against direct realism: in illusions (e.g. a stick appearing bent in water), what we perceive does not match the properties of the actual object. Since we are aware of something that differs from the object, we cannot be perceiving the object directly.
Argument from perceptual variation: An argument against direct realism: the same object appears differently to different perceivers or under different conditions. Since the object itself has not changed, what we perceive must be something mind-dependent rather than the object itself.
Argument from hallucination: An argument against direct realism: hallucinations are perceptually indistinguishable from veridical perception, yet no external object is present. If the experience is the same type in both cases, then even in veridical perception we are not directly aware of external objects.
Time-lag argument: An argument against direct realism: light takes time to travel from objects to our eyes, so we perceive objects as they were in the past, not as they are now. In extreme cases, the object may no longer exist when we perceive it (e.g. distant stars).
Solipsism: The view that only one's own mind can be known to exist. Everything else — the external world, other minds — might be merely a product of one's own consciousness. Often raised as a consequence of idealism or radical scepticism.

Reason as a Source of Knowledge

Empiricism: The view that all substantive (synthetic) knowledge is derived from sense experience. There is no innate knowledge — the mind begins as a blank slate and all its contents come from what we perceive. Associated with Locke and Hume.
Rationalism: The view that some substantive (synthetic) knowledge can be gained through reason alone, independently of sense experience. Some knowledge is innate or can be discovered by pure thought. Associated with Descartes and Leibniz.
Innatism: The view that some knowledge or concepts are innate — present in the mind from birth rather than derived from experience. Innate knowledge may be triggered by experience but is not caused by it. Defended by rationalists such as Descartes and Leibniz.
Tabula rasa: Latin for 'blank slate'. Locke's view that the mind at birth contains no innate knowledge or ideas. All knowledge is acquired through experience — the mind is like a blank sheet of paper on which experience writes.
Impressions (Hume): Hume's term for vivid, forceful perceptual experiences that we have when we see, hear, touch, or feel. Impressions are the direct products of sense experience and are more vivid than ideas. They are the raw data of experience.
Ideas (Hume): Hume's term for faint copies of impressions, produced by memory or imagination. Simple ideas are copies of single impressions; complex ideas combine simple ideas. All ideas ultimately derive from impressions — if no impression can be identified, the idea is meaningless.
Intuition (rational): It is an intellectual capacity to grasp the truth of a self-evident proposition directly and non-inferentially.
Deduction (as source of knowledge): A method of gaining knowledge by reasoning from premises to a conclusion through a chain of logical steps, where each step is guaranteed by the previous one. Descartes held that deduction, combined with intuition, is a source of a priori knowledge.
Clear and distinct ideas: Descartes' criterion for truth: an idea is clear if it is present and apparent to the mind, and distinct if it is sharply separated from all other ideas. Descartes argues that whatever we perceive clearly and distinctly must be true.

Scepticism

Philosophical scepticism: The view that knowledge is impossible, or at least that we should doubt whether we have it. Unlike everyday doubt, philosophical scepticism uses systematic doubt as a method to test what can be known with certainty.
Local scepticism: Scepticism about a particular area of knowledge, rather than all knowledge. For example, scepticism about the external world (whether we can know anything about the world outside our minds) or scepticism about other minds.
Global scepticism: Scepticism about all knowledge — the claim that we cannot know anything at all. Global scepticism is more radical than local scepticism and is often considered self-defeating, since the claim 'we know nothing' would itself have to be known.
Descartes' three waves of doubt: Three progressively more radical reasons for doubt: (1) the senses sometimes deceive us, so perhaps they always do; (2) the dream argument — we cannot be certain we are not dreaming; (3) the evil demon — a powerful deceiver could make us wrong about everything, even mathematics.
Cogito ergo sum: Latin for 'I think, therefore I am'. Descartes' foundational certainty: even if an evil demon is deceiving me about everything, the very act of doubting proves that I exist as a thinking thing. This cannot be doubted without contradiction.

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Epistemology

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