What Is Epistemology? The Theory of Knowledge, Its Questions and Thinkers

Epistemology is the branch of philosophy that studies knowledge: what it is, where it comes from, how beliefs are justified, and whether we can know anything at all. Also called the theory of knowledge, it asks what separates knowing something from merely believing it — and what entitles anyone to be confident.

You believe that the Earth orbits the Sun, that you had breakfast, and that a friend is in Lisbon because she said so. Epistemology asks which of these you know, how, and what would show you were wrong.

What does epistemology study?

The word joins the Greek epistēmē, knowledge, and logos, account; it was coined in the nineteenth century, though the questions are as old as philosophy. On The Coordinates, Root II divides epistemology into eight branches:

• Theories of knowledge — what has to be added to a true belief to make it knowledge, including the Gettier problem.
• Sources of knowledge — perception, reason and intuition, memory, testimony and revelation.
• The structure of justification — what holds a belief up, including foundationalism vs coherentism and internalism vs externalism.
• Skepticism — whether knowledge is possible, and whether you can doubt everything or only something.
• Truth — what makes a statement true.
• Rationality — what makes a belief or an action reasonable.
• Social epistemology — knowledge that depends on other people, including disagreement and expertise.
• Formal epistemology — how far evidence should move you, modelled with probability.

The 18 cards of Root II come from The Coordinates of Thought by Syed Raheel Shahzad. The card marked △ is contested: philosophers disagree about the question itself, not only about the answer.

Examples of epistemological questions

• What has to be added to a true belief to make it knowledge?
• Can you be right for the wrong reasons and still count as knowing?
• When you see a tree, what are you actually seeing?
• Do you need a reason to trust, or a reason not to?
• Is knowledge a building or a raft?
• How do you know you are not being deceived about everything?
• Should the fact that someone disagrees with you change your mind?

None is answered by gathering more facts: each asks what a fact must do to count as knowledge.

What is knowledge? Justified true belief and the Gettier problem

Two conditions are widely accepted: you cannot know what is false, and you must believe it. The hard part is the third ingredient: what turns a true belief into knowledge rather than a lucky guess?

The classic answer is justification. Knowledge is justified true belief: you know the train leaves at nine if you believe it, it is true, and you have good reason — the timetable says so. A version of this idea is discussed, and left unresolved, in Plato’s Theaetetus.

In 1963 Edmund Gettier published a three-page paper showing that this cannot be the whole story. A belief can be true and justified and still fail to be knowledge, because the justification and the truth come apart. Bertrand Russell had given an example of the same shape: you glance at a clock that has stopped, read the time, and happen to be right. Your belief is true and reasonable, yet you do not seem to know the time. Dharmottara in eighth-century India and Sriharsa in the twelfth century discussed cases like this long before.

Responses on the Gettier card include ruling out false steps in the reasoning, replacing justification with a reliable process (Alvin Goldman), requiring that the belief could not easily have been false, or, with Timothy Williamson, treating knowledge as basic and unanalysable.

Types of knowledge

Epistemology mostly studies knowing that, but there are other kinds, and a second distinction cuts across them.

Type	Example	Note
Propositional knowledge (knowing that)	Knowing that water boils at 100 °C at sea level	The main subject of epistemology
Practical knowledge (knowing how)	Knowing how to ride a bicycle	Gilbert Ryle argued it cannot be reduced to knowing facts; some philosophers disagree
Knowledge by acquaintance	Knowing a person, a city, the taste of mango	Russell contrasted it with knowledge by description
A priori	Knowing that 7 + 5 = 12	Known independently of experience
A posteriori	Knowing that it rained last night	Known through experience

Whether there is any substantive a priori knowledge is one of the oldest disputes in the field. The reason and intuition card sets rationalists, Kant, conventionalists and Mill’s empiricism side by side.

Sources of knowledge: perception, reason, memory and testimony

The familiar European story sets empiricists such as John Locke, for whom experience is primary, against rationalists such as Descartes, for whom reason can deliver truths on its own, with Kant combining the two. Indian philosophy framed the same question with unusual precision as a theory of pramāṇa, the valid means of knowledge — and the schools disagreed sharply about how many there are.

School or view	Sources of knowledge accepted
Cārvāka	Perception only; inference and testimony rejected as independent sources
Buddhist (Dignāga, Dharmakīrti)	Two: perception and inference; testimony reduces to one of these
Nyāya	Four: perception, inference, comparison and testimony, each irreducible
Mīmāṃsā	Six for the Bhāṭṭa school: the Nyāya four plus postulation and non-apprehension; five for the Prābhākara school
Strict empiricism	Sense experience only; everything else derives from it
Rationalism	Reason as well as experience; some substantive knowledge comes from reason alone
Kantian synthesis	Experience and the mind’s structure together; neither suffices alone

• Perception. When you see a tree, do you see the tree itself, as direct realists hold, or a representation your mind produces, as indirect realists hold?
• Memory. Psychologists such as Elizabeth Loftus have shown how readily memory reconstructs and distorts. Is remembering a kind of perceiving, or of imagining?
• Testimony. Hume held that trust in testimony rests on experience of its reliability; Thomas Reid that trust is the default and needs a reason to be withdrawn.
• Revelation △. Can something be known because it was disclosed rather than discovered? W. K. Clifford insisted that every belief needs sufficient evidence; William James, and later Alvin Plantinga, defended some beliefs that are not inferred from evidence.

How are beliefs justified?

Ask why you believe something, then why you believe your reason, and keep going. The ancient skeptic Agrippa pointed out that the chain must go on forever, loop back on itself, or stop at something unsupported — and none looks satisfactory. The structure of justification card lays out the replies:

• Foundationalism — the chain ends in basic beliefs that need no further support. Descartes sought indubitable foundations; modest foundationalists accept fallible ones.
• Coherentism — there is no chain; beliefs support one another in a web. Otto Neurath compared us to sailors who must rebuild their ship at sea without ever reaching dry dock.
• Infinitism — the chain is infinite, and that is acceptable (Peter Klein).
• Foundherentism — some anchoring plus mutual support (Susan Haack).

A second debate asks whether you must be able to see what justifies you. Internalists say the justifying reasons must be available to reflection. Externalists, such as reliabilists, say what matters is that the belief came from a reliable process, whether or not you can cite it. A small child who reliably recognises her mother’s voice seems to know who is calling, though she could not say how.

Skepticism: can we know anything at all?

Skepticism asks how you know you are not being deceived about everything. Descartes imagined a powerful deceiver feeding him false experiences; modern versions imagine a brain in a vat, wired to a computer. If you cannot rule that out, how can you know you have hands? The question runs through many traditions:

• Pyrrhonian skeptics such as Pyrrho and Sextus Empiricus suspended judgement — even about whether anything can be known — and held that this brought tranquillity.
• Academic skeptics such as Carneades argued, on one reading, that nothing can be known, while allowing that some impressions are more persuasive than others.
• Nagarjuna and Sriharsa used dialectic to show that rival theories of knowledge undermine themselves.
• Al-Ghazali described a period in which he doubted both the senses and reason, until certainty returned.
• Zhuangzi told of dreaming he was a butterfly, then waking unsure whether he was a man who had dreamt of a butterfly or a butterfly dreaming of a man.
• Descartes used doubt as a method, to find what could survive it.

Replies include G. E. Moore’s common-sense realism (he was surer of his hands than of any skeptical premise), fallibilism (knowledge never required certainty), contextualism (standards for “knowing” shift) and relevant alternatives (rule out only the live possibilities). Some argue that global skepticism refutes itself; Wittgenstein suggested that doubt makes sense only against a background of things not doubted.

Epistemology today: truth, evidence and trust

A good deal of current work in epistemology concerns questions anyone with a phone meets daily.

• Truth. Is a statement true because it corresponds to reality, because it coheres with other beliefs, or because it holds up in inquiry, as pragmatists such as Peirce and James proposed? Or is “true” simply a device of assertion, as deflationists following Frank Ramsey hold?
• Rationality. Should you believe what the evidence supports, or what serves you? Herbert Simon and Gerd Gigerenzer argued that good rules of thumb can be rational for limited minds.
• Social epistemology. Much of what you know depends on other people and institutions. Miranda Fricker’s work on epistemic injustice examines what happens when a speaker is believed less because of who they are.
• Disagreement. If an equally capable friend reaches the opposite conclusion, conciliationists say you should move towards her view; steadfast views say your reasons remain yours.
• Formal epistemology. Instead of asking whether you believe something, Bayesians ask how confident you should be, and how much each new piece of evidence should shift you.

Misinformation, trust in experts and AI-generated answers are, at bottom, questions about testimony and evidence.

Epistemology across traditions

Every major tradition developed a theory of knowledge.

• Greek. Plato distinguished knowledge from opinion; Aristotle analysed demonstration from first principles. See Greek and Roman philosophy.
• Indian. The Nyāya school, traced to Gautama (Akṣapāda), built a detailed theory of the pramāṇas, with careful accounts of inference and testimony. Mīmāṃsā defended the authority of the Veda. See Indian philosophy.
• Buddhist. Dignāga and Dharmakīrti founded a school of logic and epistemology that recognised only perception and inference, and took perception to be free of concepts. See Buddhist philosophy.
• Islamic. Ibn al-Haytham studied how vision works and how perception errs, and insisted on testing claims; Suhrawardi argued for a knowledge by presence that comes before concepts; Ibn Rushd held that demonstration and revelation, rightly understood, cannot conflict. See Islamic philosophy.
• Jewish and Christian. Saadia Gaon counted reliable tradition among the sources of knowledge; Maimonides and Aquinas worked out how reason and revelation relate. See Jewish and medieval Christian philosophy.
• African. Zera Yacob, in seventeenth-century Ethiopia, made reason the test of what he had been taught; H. Odera Oruka’s sage philosophy recorded the critical reflection of individual Kenyan sages. See African and Africana philosophy.
• Modern and analytic. Descartes, Locke, Hume, Reid and Kant set the modern agenda; Gettier, Goldman, Ernest Sosa and Williamson reshaped it. See modern European and analytic philosophy.

The philosophers named most often on the epistemology cards:

Epistemology vs metaphysics

Epistemology asks how we know and what entitles us to believe; metaphysics asks what is real. “Does the external world exist?” is metaphysical; “how could you know whether it does?” is epistemological. They constantly meet: how we could know about numbers depends on what numbers are.

In research-methods courses, “ontology and epistemology” name a researcher’s assumptions: what the social world contains, and how it can be known — through measurement, interpretation or both. Epistemology also borders logic, which studies which inferences are valid, and ethics, through moral epistemology and the question of whether we have a duty to believe only on sufficient evidence.

How to start studying epistemology

1. Begin with the central question. What has to be added to a true belief to make it knowledge? is on the ten-question path in Philosophy for beginners, and the Gettier problem follows directly from it.
2. Test your own beliefs. Choose something you are sure of and trace its support. Where does the chain stop?
3. Read rivals side by side. Hume and Reid on testimony, Dignāga and Nyāya on how many sources there are, Descartes and Moore on doubt.
4. Read a first book. The titles below come from the site’s Book corner, beginner-friendly ones first.
5. Join a conversation. Browse Knowledge & Truth, or ask a question of your own.

Where do you stand?

Every question here has serious answers on more than one side. Pick a card, read the rival answers, then say which you find strongest and why. Members ask, reply and take sides; reading is free for everyone.

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