Epistemology, from the Greek words episteme (knowledge) and logos (word/speech) is the branch of philosophy that deals with the nature, origin and scope of knowledge.


Definition of knowledge

Justified true belief

Plato's Theaetetus defined knowledge as justified true belief; also known as Tripartite Theory. One implication of this definition is that one cannot be said to "know" something just because one believes it and that belief subsequently turns out to be true. An ill person with no medical training but a generally optimistic attitude might believe that she will recover from her illness quickly, but even if this belief turned out to be true, on the Theaetetus account the patient did not know that she would get well, because her belief lacked justification.

Knowledge, therefore, is distinguished from true belief by its justification, and much of epistemology is concerned with how true beliefs might be properly justified. This is sometimes referred to as the theory of justification.

The Theaetetus definition agrees with the common sense notion that we can believe things without knowing them. Whilst knowing p entails that p is true, believing in p does not, since we can have false beliefs. It also implies that we believe everything that we know. That is, the things we know form a subset of the things we believe.

The problem of defining knowledge

For most of philosophical history, "knowledge" was taken to mean belief that was justified as true to an absolute certainty. Any less justified beliefs were called mere "probable opinion." This viewpoint still prevailed at least as late as Bertrand Russell's early 20th century book The Problems of Philosophy. In the decades that followed, however, the notion that the belief had to be justified to a certainty lost favour.

In the 1960s, Edmund Gettier criticised the Theaetetus definition of knowledge by pointing out situations in which a believer has a true belief justified to a reasonable degree, but not to a certainty, and yet in the situations in question, everyone would agree that the believer does not have knowledge.


Much of epistemology has been concerned with seeking ways to justify knowledge statements.


Some approaches to justifying knowledge are not rational — that is, they reject the notion that justification must obey logic or reason. Nihilism started out as a materialistic political philosophy, but is sometimes redefined as the apparently absurd doctrine that there can be no justification for knowledge claims — absurd because it appears to be self-contradictory to claim that one knows that knowledge is impossible, but perhaps for a nihilist, self-contradiction is simply unimportant.

Mysticism is the attempt to arrive at knowledge or belief through non-rational means such as faith, emotion or intuition. An instance of this may be when one bases one's belief in the existence of a thing merely on one's desire that that thing should exist. Another example might be the use of a daisy's petals and the phrase "she loves me/ she loves me not" while they are plucked to determine whether Juliet returns Romeo's affections. In both of these examples, belief is not justified through a rational means. Mysticism need not be an intentional process; one may engage in mysticism without being aware of it.


If one does not reject rationality, but still wishes to maintain that knowledge claims cannot be or are not justified, one might be termed a skeptic. Here we are on firmer philosophical ground; since skeptics accept the validity of reason, they can present logical arguments for their case.

For instance, the regress argument has it that one can ask for the justification for any statement of knowledge. If that justification takes the form of another statement, one can again reasonably ask for it to be justified, and so forth. This appears to lead to an infinite regress, with every statement justified by some other statement. It would be impossible to check that each justification is satisfactory, and so relying on such a series quickly leads to scepticism.

Alternately, one might claim that some knowledge statements do not require justification. Much of the history of epistemology is the story of conflicting philosophical doctrines claiming that this or that type of knowledge statement has special status. This view is known as Foundationalism.

One can also avoid the regress if one supposes that the assumption that a knowledge statement can only be supported by another knowledge statement is simply misguided. Coherentism holds that a knowledge statement is not justified by some small subset of other knowledge statements, but by the entire set. That is, a statement is justified if it coheres with all other knowledge claims in the system. This has the advantage of avoiding the infinite regress without claiming special status for some particular sorts of statements. But since a system might still be consistent and yet simply wrong, it raises the difficulty of ensuring that the whole system corresponds in some way with the truth.

Synthetic and analytic statements

Some statements are such that they appear not to need any justification once one understands their meaning. For example, consider: my father's brother is my uncle. This statement is true in virtue of the meaning of the terms it contains, and so it seems frivolous to ask for a justification for saying it is true. Philosophers call such statements analytic. More technically, a statement is analytic if the concept in the predicate is included in the concept in the subject. In the example, the concept of uncle (the predicate) is included in the concept of being my father's brother (the subject). Not all analytic statements are as trivial as this example. Mathematical statements are often taken to be analytic.

Synthetic statements, on the other hand, have distinct subjects and predicates. An example would be my father's brother is overweight.

Although anticipated by David Hume, this distinction was more clearly formulated by Immanuel Kant, and later given a more formal shape by Frege. Wittgenstein noted in the Tractatus that analytic statements "express no thoughts", that is, that they tell us nothing new; although analytic statements do not require justification, they are singularly uninformative.

Epistemological theories

It is common for epistemological theories to avoid skepticism by adopting a foundationalist approach. To do this, they argue that certain types of statements have a special epistemological status — that of not needing to be justified. So it is possible to classify epistemological theories according to the type of statement that each argues has this special status.


Rationalists believe that there are a priori or innate ideas that are not derived from sense experience. These ideas, however, may be justified by experience. These ideas may in some way derive from the structure of the human mind, or they may exist independently of the mind. If they exist independently, they may be understood by a human mind once it reaches a necessary degree of sophistication.

The epitome of the rationalist view is Descartes' I think therefore I am, in which the skeptic is invited to consider that the mere fact that they doubt implies that there is a doubter. Spinoza derived a rationalist system in which there is only one substance, God. Leibniz derived a system in which there are an infinite number of substances, his Monads.


Empiricists claim knowledge is a product of human experience. Statements of observations take pride of place in empiricist theory. Naïve empiricism holds simply that our ideas and theories need to be tested against reality, and accepted or rejected on the basis of how well they correspond to the facts. The central problem for epistemology then becomes explaining this correspondence.

Empiricism is associated with science. While there can be little doubt about the effectiveness of science, there is much philosophical debate about how and why science works. The Scientific Method was once favoured as the reason for scientific success, but recently difficulties in the philosophy of science have led to a rise in Coherentism.

Empiricism is often confused with positivism, which places higher emphasis on ideas about reality than on experiences with reality themselves.

Naïve realism

Naïve realism, or Common-Sense realism is the belief that there is a real external world, and that our perceptions are caused directly by that world. It has its foundation in causation in that an object being there causes us to see it. Thus, it follows, the world remains as it is when it is perceived - when it is not being perceived - a room is still there once we exit. The opposite theory to this is solipsism. Naïve realism fails to take into account the psychology of perception.


Objectivism, the epistemological theory of Ayn Rand, is similar to Naïve realism in that there is an external world, of which we gain knowledge through the senses. Objectivism holds that raw sense data is automatically integrated by the brain into percepts of entities (or objects), and that it is the function of consciousness to perceive reality, not create, invent, or alter it in any way. Once we recognize that two entities are similar to one another, and different from other objects, we are able to view them as two of the same kind of thing and form a concept which integrates all entities of that particular kind, enabling consciousness to cognitively deal with a potentially unlimited number of existents by means of a single, directly perceivable word. Objectivism rejects pure empiricism on the grounds that we are able to move beyond the level of sense-perceptions by means of objective concepts. It also rejects pure representationalism and idealism on the grounds that what we perceive is reality, and that it is meaningless to speak of a non-perceptual knowledge of reality, because percepts are our only means of gaining knowledge of reality.


Representationalism or Representative realism, unlike Naïve Realism, proposes that we cannot see the external world directly, but only through our perceptual representations of it. In other words, the objects and the world that you see around you are not the world itself, but merely an internal virtual-reality replica of that world. The so-called veil of perception removes the real world from our direct inspection.


Idealism holds that what we refer to and perceive as the external world is in some way an artifice of the mind. Analytic statements (for example, mathematical truths), are held to be true without reference to the external world, and these are taken to be exemplary knowledge statements. George Berkeley, Immanuel Kant and Georg Hegel held various idealist views.


Phenomenalism is a development from George Berkeley's claim that to be is to be perceived. According to phenomenalism, when you see "a tree" you see a certain perception of a brown shape. On this view, one shouldn't think of objects as distinct substances, which interact with our senses so that we may perceive them; rather we should conclude that all that really exists is the perception itself.

Contemporary approaches

Much contemporary work in epistemology depends on the two categories: foundationalism and coherentism.

Recently, Susan Haack has attempted to fuse these two approaches into her doctrine of Foundherentism, which accrues degrees of relative confidence to beliefs by mediating between the two approaches. She covers this in her book Evidence and Inquiry: Towards Reconstruction in Epistemology.

Reliabilism involves making predictions from what usually happens (e.g. claiming to speak Russian can be proved by a Russian speaker). There are two methods of reliable justification: External (Reliable, e.g. a doctor diagnosing me); and Internal (Unreliable, e.g. relying on sensations from my internal organs). But how do we know that something that is reliable is right? A computer program with a bug in it is reliably incorrect.

In the aftermath of the publication of the Gettier problem and other similar scenarios, a number of new definitions were formulated. While there is general consensus that truth and belief are two necessary facets of knowledge, there is a debate about what needs to be added to the true beliefs to make them knowledge, and a debate about whether justification is necessary in the definition at all.

Some examples of these new definitions include (where S is the belief holder and p is the belief):

  • ‘Applicable Knowledge’, attributed to Zacharyas Boufoy-Bastick, states that " S can know p if and only if p is true, and S is pragmatically justified in believing p.
    • This is a development in Epistemology, attained through merging Foundationism and Coherentism, which supposes that “the acceptance of belief as knowledge lies in observable and practical use”. Within the context of this theory, since an element cannot be shown to belong to the widest-possible system of beliefs, foundational knowledge is unattainable. From this clause, ‘Applicable Knowledge’ states that one must be satisfied with something less than Foundationism-based absolute knowledge and, hence, that the ‘pragmatic’ condition be added to the tripartite definition of knowledge.
  • Peter Unger's "No accident account of knowledge", which defines knowledge as "S knows p if and only if it is not at all accidental that S's belief in p is true".
  • The "Defeasibilty account of knowledge", where "There is no other proposition (q), such that if S became justified in q, S would no longer be justified in p". Under this account, q is known as the "defeater".
  • The "Causational Account", where "The fact of p causes S's belief in p"
    • A problem with the Causational account is that deviant causal chains can emerge. Philosopher Alvin Goldman added that "Fact that p, causes fact that q, causes S's belief in q is not knowledge, but belief in q, from which p is inferred, is knowledge". However, there must be an awareness of the causal chain.
  • The Conditional Account associated with Robert Nozick. S believes in p, p is the case, and if p were not the case, then S would not believe it.
  • The "Reliable Analysis" account, which adds to the "justified true belief" definition that "S arrived at p by a reliable method, or S is a reliable judge in such matters".


The problems show that there are situations in which a belief may be justified and true, and would not be knowledge. Although being a justified, true belief is necessary for a definition of knowledge, it is not sufficient. At the least, the set of our justified true beliefs contains things that we would not say that we know.

Some epistemologists have attempted to find strengthened criteria for knowledge that are not subject to the sorts of counterexamples Gettier and his many successors have produced. Most of these attempts involve adding a fourth condition or placing restrictions on the kind or degree of justification suitable to produce knowledge. None of these projects has yet gained widespread acceptance. Kirkham has argued that this is because the only definition that could ever be immune to all such counterexamples is the original one that prevailed from ancient times through Russell: to qualify as an item of knowledge, a belief must not only be true and justified, the evidence for the belief must necessitate its truth. But this conclusion is generally resisted since it easily appears to entail a sweeping skepticism.

Epistemic theories

Epistemic philosophers

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