Epistemology, the theory of knowledge, is a core component of the Western philosophical tradition.
Questions about knowledge arise in Plato, presumably inspired by the career of the historical Socrates,
and become the basis of a continuous historical dialogue in which virtually every Western philosopher has
in some way or another been engaged right down to the present day. Although these questions have
evolved over the centuries, at a very general level they remain today the same questions with which the
dialogue began. For pedagogical purposes they can be reduced to questions concerning the three
"conditions" defining knowledge:

If we start by asking "What is knowledge?" we have to set up criteria for identifying and
distinguishing "knowledge" from what is not knowledge. We assume that if we know something we also
believe whatever it that we claim to know, so the domain of "knowledge" must be a subset of the class
of "beliefs." But obviously not everything people believe is the case is in fact the case; there are false
beliefs, and so if what we believe is in fact not the case, then clearly we do not know it, although we may
falsely believe that we know it. So knowledge is made up of only those beliefs which are in fact true
beliefs. But truth cannot be the only requirement for a belief to be known, because we may believe
something and what we believe may in fact be the case, so the belief is in fact true, but our believing it
is just a matter of, let us say, a "lucky guess." The person who believes something just as a lucky guess
cannot be said to know that thing, because just guessing cannot justify the belief. Reasoning along these
lines, Plato was the first to clearly express the view that "knowledge" is "justified true belief" and this is
often called the "classical" or "traditional" definition of knowledge.

We can express this definition more formally by observing that the verb "to know" is a transitive
verb which takes a "subject" (the "knower") and a direct object (the "known"). Although there may be
much which people know which cannot be communicated in language, we will restrict our attention to
knowledge which can be expressed in language, in which case the "object" of knowledge can always be
expressed as a "proposition"; this is what we will call "propositional knowledge" or knowledge that such
and such is so. Thus we can analyze what is meant by saying that "S knows P." where "S" is any
knowing "subject" (presumably a human subject) and "P" is anything that can be known, the "object" of
knowledge, as follows. Taking its cue from Plato, the tradition has tended to identify "knowledge" with
"true, justified belief"; thus to say "S knows P." reduces to three separate claims:
 

a) S believes P (the belief condition)
b) P is true (the truth condition)
c) S is justified in believing P (the justification condition)

a) The "belief condition" implies that knowledge something that can be believed and what can be
believed is commonly identified with "statements" or "propositions". This account of knowledge thus
focuses on propositional knowledge; i.e. what can be expressed in a true or false proposition. (Other kinds
of knowledge, such as knowledge of "how" or knowledge by "acquaintance" or "description" are often
alleged to be reducible to knowledge that certain propositions are true.) Two rival accounts of "belief"
are often debated:
 

i) the dispositional view: to say S believes P means S has a disposition or tendency to behave in
a certain way

ii) the state/object view: to say S believes P means S is in a certain psychological (or "docastic")
state with respect to the object of belief, a proposition.
 

b) The "truth condition" implies that the proposition which is believed is in fact true, but deciding
whether a statement meets this condition means a theory of truth must be given. Three rival theories of
"truth" are debated:

i) the "correspondence" theory of truth: to say P is true is to say there exists a "correspondence"
relation between what P says is the case and what is really the case. This throws a great deal of
weight on giving an account of what is really the case, i.e. the nature of "reality," which is
normally taken as a metaphysical question.

ii) the "coherence" theory of truth: to say "P is true" is to say it "coheres" with an entire system
of other beliefs, it has a certain place in the totality of all truths

iii) the "pragmatic" theory of truth: to say that "P is true" means that believing P leads to the
satisfaction of certain expectations; the belief "works" or is successful in satisfying certain goals,
aims, or "interests."
 

c) The "justification condition": implies that in order for S to know P, S's believing P and P's being
true is not enough. P cannot just be a lucky guess, S has to have good reasons or reliable evidence for
believing that P is true. But to justify P by appeal to certain reasons is to say that we know those
propositions expressing those reasons, and to say S knows them requires that those reasons in turn be
justified. What are the ultimate justifiers, those propositions which are used to justify all others? Four
different views are debated:

i) infinitism: the "regress" of one proposition justifying another, which justifies another, etc., goes
on infinitely.

ii) foundationalism: by far the majority view, it holds that certain propositions are known directly
and do not need to be justified by further propositions. These propositions form the "foundation"
of all knowledge.

iii) coherentism: a proposition is justified by fitting it into a whole system of beliefs; its
justification is its part within the whole system of knowledge.

iv) contextualism: certain propositions, the ultimate justifiers, cannot themselves be justified, thus
not known; but once they are accepted, other propositions can be known by justifying them by
appeal to these ultimately unjustified propositions which form the "context" for a "world-view" or
system of knowledge. This gives a relativist view of knowledge; a proposition can be known only
relative to a particular "framework".

Obviously how a philosopher responds to one of these conditions will affect his response to others.