Epistemology is the study of the nature and scope of knowledge and justified belief. It analyzes 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. It is essentailly about issues having to do with the creation and dissemination of knowledge in particular areas of inquiry.

Epistemology asks questions like: "What is knowledge?", "How is knowledge acquired?", "What do people know?", "What are the necessary and sufficient conditions of knowledge?", "What is its structure, and what are its limits?", "What makes justified beliefs justified?", "How we are to understand the concept of justification?", "Is justification internal or external to one's own mind?"

The kind of knowledge usually discussed in Epistemology is propositional knowledge, "knowledge-that" as opposed to "knowledge-how" (for example, the knowledge that "2 + 2 = 4", as opposed to the knowledge of how to go about adding two numbers).

Knowledge is the awareness and understanding of particular aspects of reality. It is the clear, lucid information gained through the process of reason applied to reality. The traditional approach is that knowledge requires three necessary and sufficient conditions, so that knowledge can then be defined as "justified true belief":

• truth: since false propositions cannot be known - for something to count as knowledge, it must actually be true. As Aristotle famously (but rather confusingly) expressed it: "To say of something which is that it is not, or to say of something which is not that it is, is false. However, to say of something which is that it is, or of something which is not that it is not, is true."
• belief: because one cannot know something that one doesn't even believe in, the statement "I know x, but I don't believe that x is true" is contradictory.
• justification: as opposed to believing in something purely as a matter of luck.

The most contentious part of all this is the definition of justification, and there are several schools of thought on the subject:

• According to Evidentialism, what makes a belief justified in this sense is the possession of evidence - a belief is justified to the extent that it fits a person's evidence.
• Different varieties of Reliabilism suggest that either: 1) justification is not necessary for knowledge provided it is a reliably-produced true belief; or 2) justification is required but any reliable cognitive process (e.g. vision) is sufficient justification.
• Yet another school, Infallibilism, holds that a belief must not only be true and justified, but that the justification of the belief must necessitate its truth, so that the justification for the belief must be infallible.

Another debate focuses on whether justification is external or internal:

• Externalism holds that factors deemed "external" (meaning outside of the psychological states of those who are gaining the knowledge) can be conditions of knowledge, so that if the relevant facts justifying a proposition are external then they are acceptable.
• Internalism, on the other hand, claims that all knowledge-yielding conditions are within the psychological states of those who gain knowledge.

As recently as 1963, the American philosopher Edmund Gettier called this traditional theory of knowledge into question by claiming that there are certain circumstances in which one does not have knowledge, even when all of the above conditions are met (his Gettier-cases). For example: Suppose that the clock on campus (which keeps accurate time and is well maintained) stopped working at 11:56pm last night, and has yet to be repaired. On my way to my noon class, exactly twelve hours later, I glance at the clock and form the belief that the time is 11:56. My belief is true, of course, since the time is indeed 11:56. And my belief is justified, as I have no reason to doubt that the clock is working, and I cannot be blamed for basing beliefs about the time on what the clock says. Nonetheless, it seems evident that I do not know that the time is 11:56. After all, if I had walked past the clock a bit earlier or a bit later, I would have ended up with a false belief rather than a true one.

Propositional knowledge can be of two types, depending on its source:

• a priori (or non-empirical), where knowledge is possible independently of, or prior to, any experience, and requires only the use of reason (e.g. knowledge of logical truths and of abstract claims); or
• a posteriori (or empirical), where knowledge is possible only subsequent, or posterior, to certain sense experiences, in addition to the use of reason (e.g. knowledge of the colour or shape of a physical object, or knowledge of geographical locations).

Knowledge of empirical facts about the physical world will necessarily involve perception, in other words, the use of the senses. But all knowledge requires some amount of reasoning, the analysis of data and the drawing of inferences. Intuition is often believed to be a sort of direct access to knowledge of the a priori.

Memory allows us to know something that we knew in the past, even, perhaps, if we no longer remember the original justification. Knowledge can also be transmitted from one individual to another via testimony (that is, my justification for a particular belief could amount to the fact that some trusted source has told me that it is true).

There are a few main theories of knowledge acquisition:

• Empiricism, which emphasizes the role of experience, especially experience based on perceptual observations by the five senses in the formation of ideas, while discounting the notion of innate ideas. Refinements of this basic principle led to Phenomenalism, Positivism, Scientism and Logical Positivism.
• Rationalism, which holds that knowledge is not derived from experience, but rather is acquired by a priori processes or is innate (in the form of concepts) or intuitive.
• Representationalism (or Indirect Realism or Epistemological Dualism), which holds that the world we see in conscious experience is not the real world itself, but merely a miniature virtual-reality replica of that world in an internal representation.
• Constructivism (or Constructionism), which presupposes that all knowledge is "constructed", in that it is contingent on convention, human perception and social experience.

The fact that any given justification of knowledge will itself depend on another belief for its justification appears to lead to an infinite regress.

Fallibilism also claims that absolute certainty about knowledge is impossible, or at least that all claims to knowledge could, in principle, be mistaken. Unlike Skepticism, however, Fallibilism does not imply the need to abandon our knowledge, just to recognize that, because empirical knowledge can be revised by further observation, any of the things we take as knowledge might possibly turn out to be false.

In response to this regress problem, various schools of thought have arisen:

• Foundationalism claims that some beliefs that support other beliefs are foundational and do not themselves require justification by other beliefs (self-justifying or infallible beliefs or those based on perception or certain a priori considerations).
• Instrumentalism is the methodological view that concepts and theories are merely useful instruments, and their worth is measured by how effective they are in explaining and predicting phenomena. Instrumentalism therefore denies that theories are truth-evaluable. Pragmatism is a similar concept, which holds that something is true only insofar as it works and has practical consequences.
• Infinitism typically takes the infinite series to be merely potential, and an individual need only have the ability to bring forth the relevant reasons when the need arises. Therefore, unlike most traditional theories of justification, Infinitism considers an infinite regress to be a valid justification.
• Coherentism holds that an individual belief is justified circularly by the way it fits together (coheres) with the rest of the belief system of which it is a part, so that the regress does not proceed according to a pattern of linear justification.
• Foundherentism is another position which is meant to be a unification of foundationalism and coherentism.

Under the heading of Epistemology, the major doctrines or theories include: