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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. Historically, it has been one of the most investigated and most debated of all philosophical subjects. Much of this debate has focused on analysing the nature and variety of knowledge and how it relates to similar notions such as truth and belief. Much of this discussion concerns the justification of knowledge claims.

Not surprisingly, the way that knowledge claims are justified both leads to and depends on the general approach to philosophy one adopts. Thus, philosophers have developed a range of epistemological theories to accompany their general philosophical positions. More recent studies have re-written centuries-old assumptions, and the field of epistemology continues to be vibrant and dynamic.

Contents1 Definition of knowledge
1.1 Justified true belief
1.2 The problem of defining knowledge
1.3 A priori versus a posteriori knowledge
1.4 Knowledge and belief
1.5 Distinguishing knowing that from knowing how
2 Justification
2.1 Irrationalism
2.2 Rationality
2.3 Synthetic and analytic statements
3 Epistemological theories
3.1 Empiricism
3.2 Idealism
3.2.1 Nave realism
3.3 Phenomenalism
3.4 Pragmatism
3.5 Rationalism
3.6 Representationalism
3.7 Relativism
3.8 Skepticism
4 Contemporary approaches
4.1 Gettier
5 See also

Definition of knowledge

Justified true belief

In Plato's dialogue the Theaetetus, Socrates considers a number of definitions of knowledge. One of the prominent candidates is justified true belief. We know that, for something to count as knowledge, it must be true, and be believed to be true. Socrates argues that this is insufficient; in addition one must have a reason or justification for that belief.

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; and yet in the situations he describes, everyone would agree that the believer does not have knowledge.

A priori versus a posteriori knowledge

Western philosophers for centuries have distinguished between two kinds of knowledge: a priori and a posteriori knowledge.

  • A priori knowledge is knowledge gained or justified by reason alone, without the direct or indirect influence of any particular experience (here, experience usually means observation of the world through sense perception. See Rationalism, below, for clarification.)
  • A posteriori knowledge is any other sort of knowledge; that is, knowledge the attainment or justification of which requires reference to experience. This is also called empirical knowledge.

One of the fundamental questions in epistemology is whether there is any non-trivial a priori knowledge. Generally speaking rationalists believe that there is, while empiricists believe that all knowledge is ultimately derived from some kind of external experience.

The fields of knowledge most often suggested as having a priori status are logic and mathematics, which deal primarily with abstract, formal objects.

Empiricists have traditionally denied that even these fields could be a priori knowledge. Two common arguments are that these sorts of knowledge can only be derived from experience (as John Stuart Mill argued), and that they do not constitute "real" knowledge (as David Hume argued).

Knowledge and belief

Knowledge is true and believed and ...
Knowledge is true and believed and ...

There are two slightly different meanings of belief that must be distinguished. In the first sense John might "believe in" his cousin Joe. This may mean that he is willing to loan Joe money, trusting in his paying it back. In this sense, John might say, "I know it is safer to fly than drive, yet I don't believe it" in which case John doesn't trust in the pilots of commercial aircraft, even though as a cognitive matter he may understand the pertinent statistics.

In the second sense of belief, to believe something just means to think that it is true. That is, to believe P is to do no more than to think, for whatever reason, that P is the case. It is this sort of belief that philosophers most often mean when they are discussing knowledge. The reason is that in order to know something, one must think that it is true - one must believe (in the second sense) it to be the case.

Consider someone saying "I know that P, but I don't think P is true". The person making this utterance has, in a profound sense, contradicted themselves. If one knows that P, then, amongst other things, one thinks that P is indeed true. If one thinks that P is true, then one believes P. (See: Moore's paradox.)

Knowledge is distinct from belief and opinion. If someone claims to believe something, they are claiming that they think that it is the truth. But of course, it might turn out that they were mistaken, and that what they thought was true was actually false. This is not the case with knowledge. For example, suppose that Jeff thinks that a particular bridge is safe, and attempts to cross it; unfortunately the bridge collapses under his weight. We might say that Jeff believed that the bridge was safe, but that his belief was mistaken. We would not say that he knew that the bridge was safe, because plainly it was not. For something to count as knowledge, it must be true.

Similarly, two people can believe things that are mutually contradictory, but they cannot know (unequivocally) things that are mutually contradictory. For example, Jeff can believe the bridge safe, while Jenny believes it unsafe. But Jeff cannot know the bridge is safe and Jenny cannot know that the bridge is unsafe simultaneously. Two people cannot know contradictory things.

Distinguishing knowing that from knowing how

Suppose that Fred says to you: "The fastest swimming stroke is the front crawl. One performs the front crawl by oscillating the legs at the hip, and moving the arms in an approximately circular motion". Here, Fred has propositional knowledge of swimming and how to perform the front crawl.

However, if Fred acquired this propositional knowledge from an encyclopedia, he will not have acquired the skill of swimming: he has some propositional knowledge, but does not have any procedural knowledge or "know-how". In general, one can demonstrate know-how by performing the task in question, but it is harder to demonstrate propositional knowledge. Michael Polanyi popularised the term tacit knowledge to distinguish the ability to do something from the ability to describe how to do something. Gilbert Ryle had previously made a similar point in discussing the characteristics of intelligence. His ideas are summed up in the aphorism "efficient practice precedes the theory of it". Someone with the ability to perform the appropriate moves is said to be able to swim, even if that person cannot precisely identify what it is they do in order to swim. This distinction is often traced back to Plato, who used the term techne or skill for knowledge how, and the term episteme for a more robust kind of knowledge in which claims can be true or false.


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.

One definition of contemporary Mysticism is the use of non-rational methods to arrive at beliefs and accepting such beliefs as knowledge. For example, believing that something is true based on emotion may be regarded as epistemological mysticism. An instance of this may be when one bases one's belief in the existence of something merely on one's desire that it should exist. Another example might be the use of a daisy's petals and the phrase "he loves me / he loves me not" while they are plucked to determine whether Romeo returns Juliet's affections. The contemporary mysticism in this example would be the assumption that such a method has predictive or indicative powers without rational evidence of such (This does not necessarily lessen its importance as a symbolic tool in human thought). In both of these examples, belief is not justified through a rational means. Mysticism need not be an intentional process: one may engage in contemporary mysticism without being aware of it. However, contemporary Mysticism should not be confused with traditional Mysticism, which is not an assumption of knowing but a spiritual practice in many eastern traditions. In this case, it is the practice of focusing thought, not the content of the thought which is important.


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 that statement also 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 skepticism.

Yet, others such as Peter Klein have argued that a regress isn't malignant. One argument is that the so called infinitism of justification is an acceptable aspect of knowledge because having an infinite amount of justifications is the only possible way to answer the doubt ad infinitum of skeptics. Furthermore, some justified reasons for belief aren't always relevant or apparent while one still has knowledge, thus, one needn't worry about checking all of one's infinite justifications. An agent may remain unaware of the justifications so long as there's the possibility of having them for one's statement and they're in fact true.

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. W.V.O. Quine, in his famous Two Dogmas of Empiricism, challenged the legitimacy of the analytic-synthetic distinction altogether.

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.


Empiricists claim knowledge is a product of human experience. Statements of observations take pride of place in empiricist theory. Nave 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 observed 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 recent difficulties in the philosophy of science have led to a rise in coherentism.

Empiricism is sometimes associated with a tradition called logical empiricism, or positivism, which places higher emphasis on ideas about reality rather than on experiences of reality.


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. Idealism is itself a metaphysical thesis, but has important epistemological consequences.

Nave realism

Nave realism, sometimes called 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. Nave realism fails to take into account the psychology of perception. (See: G.E. Moore.)


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, when you touch it, you get a perception of pressure against your palm. 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.


Pragmatism about knowledge holds that what is important about knowledge is that it solves certain problems that are constrained both by the world and by human purposes. The place of knowledge in human activity is to resolve the problems that arise in conflicts between belief and action. Pragmatists are also typically committed to the use of the experimental method in all forms of inquiry, a non-skeptical fallibilism about our current store of knowledge, and the importance of knowledge proving itself through future testing.


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' Cogito ergo sum ("I think, therefore I am"), in which the skeptic is invited to consider that the mere fact that he doubts this claim implies that there is a doubter. Because doubting is a kind of thinking, the claim must be correct. 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.


Representationalism or representative realism, unlike nave 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.


Relativism as advocated by Protagoras maintains that all things are true and in a constant state of flux, revealing certain aspects of truth at one time while concealing them at another. It claims that there is no objective truth: anything which a person can perceive is true for that person, but not necessarily true for the next person. By equating perceptions and beliefs with truth, overt self-contradiction is avoided.


When scientists or philosophers ask "Is knowledge possible?", they mean to say "Am I ever sufficiently justified in believing something in order to have knowledge?" Adherents of philosophical skepticism often say "no". Philosophical skepticism is the position which critically examines whether the knowledge and perceptions people have are true; adherents of this position hold that one can never obtain true knowledge, since justification is never certain. This is a different position from scientific skepticism, which is the practical stance that one should not accept the veracity of claims until solid evidence is produced.

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.


Main articles: Edmund Gettier & Gettier problem

Edmund Gettier argued that there are situations in which a belief may be justified and true, and yet would not count as knowledge. Although being a justified, true belief is necessary for a statement to count as 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. Though this seems to imply a sweeping skepticism, Kirkham notes that it doesn't exclude the possibility of rational belief altogether.

Gettier's article was published in 1963. Right after that, for a good decade or more, there were an enormous number of articles trying to supply the missing fourth condition of knowledge. The big project was to try to figure out the "X" in the equation, Knowledge = belief + truth + justification + X. Whenever someone proposed an answer, someone else would come up with a new counterexample to shoot down that definition.

Some of the proposed solutions involve factors external to the agent. These responses are therefore called externalism. For example, one externalist response to the Gettier problem is to say that the justified, true belief must be caused (in the right sort of way) by the relevant facts.

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.

See also