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Epistemology (from επιστήμη, knowledge) is the branch of philosophy that examines how human beings acquire knowledge and form beliefs, the nature of knowledge and beliefs, and how they differ from each other.
 Definitions of knowledge
Justified true belief (JTB) is one of the most common definitions of knowledge. It states that knowledge is:
- a belief: one can't know something if they don't believe it
- which is true: one cannot "know" 1=2, right?
- and justified: if one believes that there is a block of cheese orbiting Jupiter, and if it so happens that there is a block of delicious cheese orbiting Jupiter, that person still can't be said to have known that because they had no reason for their belief - they just lucked out that it was true.
The JTB model of knowledge is often sourced to Plato, who sought to ascertain the distinction between knowledge and mere true opinion. He identified justification as the key element. Various versions of the JTB model have been advanced by many subsequent philosophers. For the most part, the various versions differ on what constitutes justification.
The JTB model is not unquestioned. At some time or other all three elements of the JTB model have come under fire. David Lewis, for instance, maintains that knowledge requires neither belief nor justification. One notable objection to the JTB model of knowledge is the so-called "Gettier problem" or "Barn problem": Suppose a man looks at a large field, and he sees a cardboard cutout of a barn in that field. The barn facade resembles a real barn in every particular. Suppose further that there is a barn in the field, but so far away he couldn't possibly see it, or maybe behind a hill or something. Then the man will believe that there is a barn in the field, and his belief will be both true and justified (since he has reason to believe that there is a barn, and there is a barn). Nonetheless we are not inclined to say that he knows that there is a barn, since his justification is not appropriately related to his belief. Thus we have a counterexample to the JTB model.
 Problems In Epistemology
- What is knowledge? In particular, what distinguishes knowledge from true belief?
- What is the object of knowledge? Typically, knowledge is understood as a relation between a knower and a proposition, but perhaps this is not the best way to understand things.
- Does knowledge require justification If so, what is required in order for a belief to be justified?
- Do we know anything at all? We think we know all kinds of things, but perhaps we are mistaken. This is the problem of skepticism. There are various kinds of skepticism, from the global skepticism of Pyrrhonism to various kinds of local skepticism. A person might, for instance, be a local skeptic about ethical claims, which would entail believing that we cannot know anything about ethical claims.
- Is knowledge closed? If so, what is it closed under?