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Epistemology or theory of knowledge is the branch of philosophy that studies the nature, origin, and scope of knowledge and belief. The term "epistemology" is based on the Greek words, "ἐπιστήμη or episteme" (knowledge or science) and "λόγος or logos" (account/explanation); it was introduced into English by the Scottish philosopher, James Frederick Ferrier.

Much of the debate in this field has focused on analyzing the nature of knowledge and how it relates to similar notions, such as truth, belief, and justification. It also deals with the means of production of knowledge, as well as skepticism about different knowledge claims. In other words, epistemology primarily addresses the following questions: "What is knowledge," "How is knowledge acquired," and "What do people know?"

The two central questions of epistemology could be summed up as: What do humans know and how do they know it? Some of the numerous subsidiary questions include: What is knowledge? Are there different types or kinds of knowledge? Does the process of knowing differ in different domains of human knowledge and activity? Does what is knowable differ in different domains? Does genuine knowledge require justification or evidence? Must one believe something in order to know it? Is there a difference between knowledge and true belief, and if so what is it? Can one know a statement or proposition S if S is false?

The field of epistemology in western philosophy is extremely broad. From the ancient Greeks Plato and Aristotle to today, probably more has been written on this topic, broadly understood, than on any other branch of philosophy.


Many other topics fit under or are part of the broad heading of epistemology. Some of those are: analytic and synthetic statements, a priori and a posteriori statements, categories, certainty, concepts, the correspondence and coherences theories of truth, common sense, criterion, doubt, empiricism, error, experience, idealism, ideas, innate ideas, intentionality, intuition, irrationalism, memory, phenomenology, philosophy of science, pragmatism, presuppositions, judgments, statements, rationalism, realism, sensationalism, thinking, and universals.

Definition of knowledge

The problem or question of a definition of knowledge turns out to be much more difficult and controversial than it may initially seem to be. Nearly every philosopher who has written on the topic has a somewhat different understanding and definition—whether this is tacit or explicit—of knowledge. Broadly speaking, at least until the middle of the nineteenth century, most philosophers and philosophical approaches to the question can be divided into empiricists (or empiricism) and rationalists (or rationalism), with empiricists stressing the role of sensory perception of the sensible world in acquiring knowledge, and rationalists stressing the role of mind and its activity; the rise of pragmatism, phenomenology, and some other twentieth century philosophical schools and movements may mean that no longer can all theories of knowledge can be subsumed under rationalism and empiricism.

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 twentieth century book, The Problems of Philosophy. In the decades that followed, however, the notion that the belief had to be justified to a certainty began to crumble.

Some philosophers have said that knowledge is simply, "Awareness of something" (Professor Sebastain Matczak, in an unpublished class presentation).

Plato distinguished between knowledge and true belief. A juror, for example, may believe on the basis of testimony he has heard in court that some proposition, "P" is true (for example, that Smith committed the murder for which he is charged), and "P" may in fact be true (that is, Smith actually committed the murder) so this juror has a true belief; but the juror did not observe the murder taking place, so—according to Plato—he does not have genuine knowledge that Smith did it, whereas an eyewitness to that murder has genuine knowledge. Plato discussed knowledge in several of his dialogues, Theatetus being the most prominent. His conclusion there is that knowledge consists of true belief plus an account or key—a justification; the term he used for this is a logos.

Bertrand Russell distinguished between knowledge by description and knowledge by acquaintance. Most people, for example, gain knowledge of scientific facts or data—of rare plants or animals, for example—by description, by reading accounts of these things in science texts. So that knowledge could be called less secure or less certain than that of a small number of people—especially the scientists who make the discoveries and who then go on to write about them and describe them for others—have direct experience of such animals or plants, so their knowledge is by acquaintance. Nowadays, for another example, because the participants have all died, the knowledge that anyone has about World War I is by description, but while they lived, some people who had participated in World War I had knowledge of it (or some part of it) by acquaintance.

Another important distinction is that between knowing that and knowing how. Suppose that Fred says to his friend: "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. Gilbert Ryle had previously made this 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.

Michael Polanyi went farther than this. He argued that explicit knowledge arises out of and rests on a background of what he called "tacit knowledge" and "the tacit dimension." He (and others) claimed that knowing is a skill, and that skills are learned but that they are (usually) not explicit or specifiable.

Knowledge and belief

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 lend 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 safety of 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 the view of most philosophers, 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 seems to have contradicted himself. 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.

Knowledge is usually held to be 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. One might say that Jeff believed that the bridge was safe, but that his belief was mistaken. One 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—at least as knowledge is usually understood anyway.

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

Justified true belief

Plato, and many others following him, defined knowledge as justified true belief. Plato dealt with knowledge in several of his dialogues, with the Theatetus being the most explicit.

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 has been 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 one can believe things without knowing them. While knowing "p" entails that "p" is true, believing in "p" does not, since one can have false beliefs. It also implies that people believe everything that they know. That is, the things people know form a subset of the things they believe. But that claim, too, is questionable.

Is the justified true belief theory true?

There are good reasons to think that at least two of the conditions of the justified true belief theory—and possibly all three of them—are false or at least highly suspect.

Consider the woman who knows that her husband is cheating on her, but refuses to believe it. At some level, she knows it is true that he is committing adultery, but she also refuses to believe it, so she could be said, correctly, not to believe what she knows. Similar things could be said about the parent who knows that her child is using drugs but refuses to believe it, or the businessperson who knows that one of his associates is embezzling money from the firm but does not allow himself to believe it. This suggests that belief is not necessary for knowledge.

Many people have pointed out that knowledge does not require justification. As we will see below, justificationism itself breaks down for numerous reasons, including the infinite regress problem. But also, as Polanyi pointed out, we all know more than we can prove, so proof or justification is not essential for knowledge.

A third problem concerns truth. If one cannot know something unless it is true, then what about the leading scientists of the past whose hypotheses and theories were based on the best evidence they had at the time and held the field for a time but were later overthrown? Did they not know anything? Is there no such thing as scientific knowledge, since all scientific knowledge is subject to being overthrown or discarded because of what is discovered in the future?

Moreover, in the 1960s, Edmund Gettier criticized the justified true belief 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 in question, everyone would agree that the believer does not have knowledge. Consider, for example (this is not Gettier's example, but it is sufficiently similar to make the point), the person who believes that there is a sheep in the field in front of him. He has based this belief on his seeing a dog in the field and mistaking that dog for a sheep. But, unseen by him, there really is a sheep in the field, so his belief is true. Thus he has a justified true belief that there is a sheep in the field. But because his belief and justification are based on a mistake, most people would agree that he does not have genuine knowledge that there is a sheep in the field.

A priori versus a posteriori knowledge

Western philosophers, especially following the work of Immaneul Kant, 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).

A posteriori knowledge is any other sort of knowledge; that is, knowledge that is attained or justified by reference to sensory 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. Plato, for example, pointed out that we have the notion of and knowledge about absolute equality, but that anything we can perceive through our senses, such as two sticks, cannot be absolutely equal. Thus, he concluded, our knowledge of absolute equality cannot be gotten through sense experience.

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).


Much of epistemology has been concerned with seeking ways to justify knowledge statements. As Walter B. Weimer has written:

The majority of contemporary and historical thinkers—whether philosophers, scientists, theologians, artists, or whatever—share the essentials of a metatheory of rational inquiry and behavior that … I call justiticationism. …the common deep structure of [justificationism underlies] the chaotic diversity of "received view" positions in philosophy, theology, art, even the theory of rationality and rational inquiry itself (Notes on the Methodology of Scientific Research, Chap. 1).

Weimer went on to claim that all the traditional philosophical theories or "world views" such as empiricism, rationalism, or idealism are surface-level variations on this deep-level metatheory of justificationism.

Two cardinal traits of justificationism are that it ties together knowledge and proof and that it bases or grounds knowledge in some form of authority. W.W. Bartley III described that authoritarianism in this way:

The western philosophical tradition is authoritarian in structure, even in its most liberal forms. This structure has been concealed by oversimplified traditional presentations of the rise of modern philosophy as part of a rebellion against authority. In fact modern philosophy is the story of the rebellion of one authority against another authority, and the clash between competing authorities. Far from repudiating the appeal to authority as such, modern philosophy has entertained only one alternative to the practice of basing opinions on traditional and perhaps irrational authority: namely that of basing them on a rational authority.

This may be seen by examining the main questions asked in these philosophies. Questions like: How do you know? How do you justify your beliefs? With what do you guarantee your opinions—all beg authoritarian answers—whether those answers be: the Bible, the leader, the social class, the nation, the fortune teller, the Word of God, the intellect, or sense experience (The Retreat to Commitment, 134, 135).

Justificationism broke down, however, when people realized that no scientific proposition is provable or can be known with certainty. In 1828, Jakob Friedrich Fries showed that logical relations such as provability, consistency, and so on apply only between propositions and propositions can be derived only from other propositions, not from "facts," where "fact" means some extra-linguistic state of affairs (Neue oder anthropologische Kritik der Vernunft). The work of Pierre Duhem undermined the notion of eternal facts as a basis for theory when he showed that science is fact correcting, rather than fact preserving, because new theories often refute the "facts" of older theories and give rise to new "facts" (The Aim and Structure of Physical Theory). The work of Fries, Duhem, and others undermined justificationism because it destroyed the factual basis as an authority on which justification was supposedly erected.

That classical form of justificationism was then modified into neojustificationsim, which changed "proof" to "probable," meaning that the most probable statement or theory was to be selected. This led to a lot of work toward developing probabilistic and inductive logic. The goal was to to develop a "confirmation theory" that would assign a probability rating for a hypothesis on the basis of some body of evidence, stated as P(h,e), where the probability would lie in the range of 0<p<1, with 0 meaning complete improbability and 1 being complete certainty.

Neojustificationism, however, also met its downfall when there was found no justification of induction—a problem that had been expressed clearly by David Hume. Many attempts to justify induction were made and the effort continues, but none of these efforts has succeeded.

Irrationalism, anti-foundationalism, skepticism, mysticism

Some responses to the problem of 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—apparently absurd because it appears to be self-contradictory to claim that one knows that certain knowledge is impossible. The paradox here, however, is that one can know that all attempts to ground or find an unassailable starting point or method for acquiring knowledge are doomed to failure because all such supposedly certain or unassailable starting points or methods can be questioned and shown to be less than certain; thus one can know that one does not ever know for certain. Some call this position irrational, but it is more accurate to call it Anti-foundationalist.

Skepticism is the view that nothing is or can be known for certain. The skeptical attitude or stance can be either destructive or constructive. In the hands of Socrates and anyone with a Socratic attitude, the stance is constructive because it seeks to ascertain just what one knows and how one knows it. In the hands of the nihilist or cynic, it is usually destructive because it assumes that all voices and claims and purported knowledge statements are bogus and, thus, equally worthless and unworthy of belief or genuine examination.

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 would be regarded as epistemological mysticism, whereas believing based on deductive logic or scientific experiment would not. 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 mysticism in this example would be the assumption that such a method has predictive or indicative powers without rational evidence of such. 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 non-justificationist. Here philosophers are on firmer philosophical ground; since non-justificationists 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.

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 mention 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 attempt 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 he doubts this claim implies that there is a doubter. Notice, however, that Descartes' Cogito is based on prior acceptance of a substance-accident ontology, whereby an accident (thinking, in this case; doubting is itself a form of thinking) cannot exist unless there is a substance (I, as the doubter) in which that accident can inhere. Because doubting is a kind of thinking, the claim must be correct, given the prior acceptance of the substance-accident ontology. Empiricists, however—especially Hume—rejected the necessity of that ontology. 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.

Rationalist epistemologies usually adopt some form of coherence theory of truth.


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 observed facts; this is usually called the correspondence theory of truth. 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 favored as the reason for scientific success, but recently difficulties in the philosophy of science have led to a rise in Coherentism; that is, in a movement toward a coherence theory of truth.

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 people exit. The opposite theory to this is solipsism. But 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 it holds that there is an external world, of which people 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 people recognize that two entities are similar to one another, and different from other objects, they 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 people perceive is reality, and that it is meaningless to speak of a non-perceptual knowledge of reality, because percepts are people's only means of gaining knowledge of reality.


Representationalism or Representative realism, unlike Naïve Realism, proposes that people cannot see the external world directly, but only through perceptual representations of it. In other words, the objects and the world that can be observed 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 direct inspection. This is similar to Immanuel Kant's claim that the "thing in itself" is unknown and unknowable because human knowledge of it is always conditioned by perceptual process.


Idealism holds that what people 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.


Phenomenalism is a development from George Berkeley's claim that to be is to be perceived. According to phenomenalism, when people see a tree, they see a certain perception of a brown shape, when people touch it, you get a perception of pressure against their palm. On this view, one shouldn't think of objects as distinct substances, which interact with senses so that people may perceive them; rather people 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 the current store of knowledge, and the importance of knowledge proving itself through future testing.

Anti-certainty views

Closely related to pragmatism are claims or observations that none of humanity's knowledge—except knowledge within a formal system—is certain or foundational. One philosopher who emphasized this point was Karl Popper. Popper used the metaphor of piles driven into a swamp to express this view about human knowledge. The piles are driven down until people can erect their edifice (of knowledge) upon them, but they never reach bedrock, that is, all knowledge—even that which seems to be most certain or least subject to challenge—is subject to question and revision if we find some need or reason to do so. Following the metaphor, if the piles turn out to be unable to support the edifice built on them, they are pounded down further, but it is never knowmn for certain that they will not need to be pounded down even further.

This applies to scientific knowledge, observations, and any purported knowledge about any subject, except for formal knowledge that comes from a formal system (logic or mathematics).

Religious believers who claim that God's word or God's truth is certain face the same problem—there have been times in the past when people had to revise or change what they previously thought was certain knowledge coming from God, so supposed knowledge coming in this way or from this source is never certain either.

The only kind of knowledge to which this uncertainty does not apply is knowledge within a formal system. It is known for certain within formal logic, for example, that if "P" and "If P, then Q" are both true, then "Q" must necessarily be true (the logical principle known as Modus Ponens). Or if "P" is true and "Q" is true, then "P and Q" is true (the logical principle known as Addition. Within geometry, it is known for certain that, if one is on a Euclidian plane, the sum of the internal angles of a triangle is exactly equal to the sum of two right angles. The certainty that can be found within formal systems, however, is a function of the postulates and derivation rules that we posit as the beginning points of logical or geometric or axiomatic systems. So this is still a conditional certainty, and if one found some reason to revise even those fundamental rules of logic one would do so. (This point about the possibility that even fundamental rules of logic may be subject to revision was made by Willard Quine.)


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 one's justified true beliefs contains things that one would not say that one knows.

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.

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.

It could be pointed out that all the attempts to deal with the Gettier problem seem to arise from a belief that knowledge, in order to be real or genuine, must be certain and must rest on some unassailable foundation. But that is not necessarily the case if one accepts the Popperian view that no knowledge is certain—it is all just an edifice that is erected on an insecure foundation with any and all parts subject to revision if revision becomes warranted or desirable. If one accepts that point, then the distinction between knowledge and faith is not so strong or bright-line after all.

Present day approaches

Much present day work in epistemology depends on attempts to reinstate and build on the notions of foundationalism (the notion that there must be foundations for knowledge) and coherentism (that is, a coherence theory of truth).

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.

Epistemic theories

Epistemic philosophers


  • Clay, Marjorie, and Keith Lehrer (eds.). 1989. Knowledge and Skepticism. Boulder, Colo.: Westview Press. ISBN 0813307783.
  • Greco, John, and Ernest Sosa (eds.). 1999. The Blackwell Guide to Epistemology. Malden, MA: Blackwell Publishers. ISBN 0631202900.
  • Haack, Susan. 1995. Evidence and Inquiry: Towards Reconstruction in Epistemology. Oxford: Blackwell. ISBN 0631118519.
  • Norris, Christopher. 2005. Epistemology. London: Continuum. ISBN 0826477313.
  • Rand, Ayn, edited by Leonard Peikoff and Harry Binswanger. 1990. Introduction to Objectivist Epistemology. Expanded 2nd. Ed., New York: New American Library. ISBN 0452010306
  • Richard Kirkham. 1984. "Does the Gettier Problem Rest on a Mistake?" In Mind, 93.
  • Steup, Matthias and Ernest Sosa, Eds. 2005. Contemporary Debates in Epistemology. Malden, MA: Blackwell Pub. ISBN 1405107383 ISBN 1405107391

External links

All links retrieved September 24, 2013.

General philosophy sources


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