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Epistemology (from Greek ἐπιστήμη, or episteme, knowledge; and λόγος, or logos, a study or account) is also called theory of knowledge. A philosophical discipline, epistemology deals with broad questions concerning cognition: what is knowledge? Is certainty required for knowledge, and what is certainty anyway? From what sources—sense-perception, say, or revelation—do we derive knowledge? Our beliefs can have more or less justification, warrant, or evidence—and these features seem, roughly speaking, required for knowledge. So what are they? Some thinkers doubt that we have any, or very much, knowledge at all. On what grounds can we embrace, or reject, such skepticism?

Philosophers have virtually no unanimous views on these topics. Still, to get the lay of the land, it will help to look at some common opinions. Many, perhaps most, philosophers hold that knowledge is something like justified, true belief; that certainty is not actually required for knowledge; that our basic sources of knowledge include at the very least sense-perception, memory, and reasoning; that our beliefs are justified, ultimately, by a foundation of obvious, self-justifying beliefs, or by their mutual coherence, or by being the result of reliable belief-forming processes; and finally, that we are capable of having some knowledge. These are just some popular, sample views. They are very far from being universally held.

Occasionally, certain other topics are included in epistemology, particularly the philosophy of perception as well as questions about philosophy of language, and even logic. This article, however, is focused on the earlier-listed questions.

The nature of knowledge

Sometimes we speak as if knowledge is no more than true belief. But epistemologists (as they are called) are nearly united in their scorn of "true belief" as a definition of knowledge. Plato famously proposed that knowledge is true belief "with the addition of an account (logos)."[1] An "account" is something like a reason, evidence, or justification, and this seems necessary because of the following sort of situation. Suppose you're playing roulette, and you're a superstitious type of person: you think that, if you believe hard enough that the ball lands on 7, then the ball will land on 7. This is, we can stipulate for the example, complete nonsense. So you believe with all your might that the ball will land on 7, and lo, it lands on 7. You had a true belief, but not knowledge. Surely you didn't know, in any strict sense, what number the ball would land on—i.e., your 'true belief' was not 'justified'. Epistemology studies this strict sense of knowledge.


Epistemologists are also concerned to understand what it takes for a belief to be justified. One central question is whether we need to have reasons for thinking our justified beliefs true, and whether we need to be aware of those reasons. Coherentists answer the first question in the affirmative, and further add that these reasons must be additional justified beliefs which will in turn depend for their justification on other justified beliefs; they are opposed by foundationalists, who argue that some beliefs can be justified without depending on other beliefs. Some accessibilist internalists also answer the second question in the affirmative. But both of these answers are the subject of much dispute.


  1. Plato, Theatetus, 201d. Actually, Plato has Theatetus report something that he remembered: "I had forgotten but now it comes back to me. He said that true belief with the addition of an account was knowledge, while belief without an account was outside its range." Later in the dialogue, characteristically, Socrates finds grounds on which to reject this definition.

Further Reading

Classical works

Contemporary works