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Epistemology is the branch of philosophy that deals with the nature, origin and scope of knowledge.

Table of contents
1 Definition of knowledge
2 Justification
3 Epistemological theories
4 Contemporary approaches
5 References
6 External links and references

Definition of knowledge

Justified true belief

The most influential attempt to define knowledge can be traced back to
Plato’s Theaetetus. This defined knowledge as justified true belief.

One implication of this definition is that one can't be said to "know" something just because one believes it and that belief turns out to be true. An ill person with no medical training but a generally optimistic attitude might believe that he will recover from his illness quickly. This might turn out to be true - and there might be a connection between the truth and the belief in it, if the patient's optimistic attitude helped bring about this confirmation. Still, most philosophers today would probably say that the patient did not have knowledge that he would get well before he did - because there was not enough justification, or warrant, for the belief.

In the context of epistemology, belief is not used in the sense of having confidence or faith in something. Belief is used in the sense of assenting to the truth of some proposition or statement. Beliefs in this sense are either true or false. If Jenny believes that x is true, and x is in fact true, then Jenny holds a true belief. But on the Theaetetus account, if that belief is to count as knowledge, it must also have a suitable 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, philosophers came to think of knowledge as meaning "justified true belief," and the notion that the belief had to be justified to a certainty was forgotten. In the 1960s, Edmund L. Gettier refuted this careless 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.

The problems show that there are situations in which a belief may be justified and true, and yet most would not consider it to 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.

So epistemologists have attempted to find strengthened criteria for knowledge that will not be subject to the sorts of counterexamples Gettier and his many successors have produced. No one has yet succeeded in doing that. Kirkham (see the References section below) 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 resisted since it would probably entail a sweeping skepticism.


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 justifies its statements by claims of direct experience of the divine. Those fortunate enough to have had such experiences again might be untroubled by any perceived lack of coherence in their statements, since it might simply be impossible for a human mind to comprehend an experience of the divine in a coherent fashion.


If one does not reject rationality, but still wishes to maintain that knowledge claims can not 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) includes 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 there are innate ideas that are not found in experience. These ideas are justified independently of any experience people may have. 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. Naive 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.

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.

Na´ve realism

Na´ve realism, or Common-Sense realism is the most straight-forward theory of perception. It has its foundation in causal relation 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. Most would condemn this view as flawed due to the myriad ways in which an object, or sound, may be interpreted - a table may look skewed at various angles.

Representative realism

Representative realism does, unlike Na´ve Realism, take into account sense data (the way in which the object is interpreted, not simply the objective, mathematical object) - this induces the veil of perception wherein we are unsure the table we look at exists due to there being no objective proof of existence.


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 known to be the case without reference to the external world, and these are taken to be exemplary knowledge statements. George Berkeley, Immanuel Kant and George 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 .

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

In the aftermath of the publication of the Gettier problems 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 debate about what needs to be added to true beliefs to make them knowledge, and 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):


Edmund Gettier, "Is Justified True Belief Knowledge?" Analysis, 23, 1963. Richard Kirkham, "Does the Gettier Problem Rest on a Mistake?" Mind, 93, 1984. Bertrand Russell, The Problems of Philosophy,

Epistemic theories

Epistemic philosophers

Related topics

External links and references