The branch of philosophy that deals with the nature of knowledge.
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One might wonder: What do I have to do, to be sure that I do have the truth? How can I be sure that my beliefs are true? Is there some sort of guarantee available to me -- some sort of criterion I might use, in order to decide, as rationally and as carefully as I possibly could, that indeed what I believe is true?
Suppose you thought your belief had been arrived at rationally. You used logic, you based your belief on observation and experiment, you conscientiously answered objections, and so forth. So you conclude that your belief is rational. Well if so, then your belief has at least some claim to be true. Rationality is a indicator of truth: if your belief is rational, then it is at least probably true. At the very least, the rationality of a belief is reason to think the belief is true.
A feature of belief is an epistemic feature if it is at least some indication that the belief is true.
Many of our beliefs do, I think, have lots of positive epistemic features; many of our beliefs are quite rational, quite justified, very probably true, highly warranted, and so on. But most of us, I think, at least in some moments, don't want to rest content with just being rational. We don't want to have a rational belief that is, unfortunately, false. Because that's possible, right? I can be very conscious, careful, and logical in forming a belief, and so be rational in holding the belief; but it still might be false. So rationality isn't our ultimate ambition that we have for our beliefs.
Our ultimate ambition for our beliefs is knowledge. Because if I do know something, then not only am I justified, or rational, in a belief; because I have knowledge, I have the truth. So naturally, when we are thinking about the epistemic features of our beliefs, the big question is this: When do I have knowledge? When can I say that I have it? As I'm sure you are aware, there are some people who claim that we can't have knowledge; such people are called skeptics. More on that, of course, later.
Now I can describe to you the field of epistemology, which is also called the theory of knowledge. Here is a definition:
Epistemology includes the study of (1) what the epistemic features of belief, such as justification and rationality, each are (e.g., what justified belief is); (2) the origin or sources of such features (and thus the sources of knowledge); (3) what knowledge is, i.e., what epistemic features would make a true belief knowledge; (4) whether it is possible to have knowledge.
So, first, epistemologists spend a great deal of time concerning themselves with various epistemic features of belief, such as justification and rationality. And they write long articles and books trying to say just when beliefs are justified, or rational.
A second, related concern is where such epistemic features ultimately come from. If I say, for example, that my belief that Paris is the capital of France is justified, I can ask: Where did the justification for my belief come from? Probably at some point some reliable source told me that Paris is the capital of France; and that was enough to make me justified in adopting the belief. OK, then one, but only one, source of justification would be testimony, which is just a fancy word for what other people tell me. Another source of justification would be sense-perception. So epistemology asks: What are the ultimate sources of justification, rationality, or other epistemic features of belief? And that allows to answer a further question: What are the ultimate sources of knowledge?
Which brings us to the third topic studied by epistemologists, namely, what knowledge is. The question here isn't what we can know, or even what we do know. The question is: What would knowledge be, if we had it? A belief has to pass some sort of muster if it's to count as knowledge. So what features would a belief have to have, in order to be an actual piece of knowledge -- not just something that pretends to be knowledge, but which is actually knowledge?
Then, fourth, there is one of the more difficult topics of philosophy -- trying to answer, or otherwise deal with, the challenge that we cannot have knowledge. A number of philosophers -- not too many, but some -- have said that we cannot have knowledge. A lot of philosophers have said that it's very difficult to obtain knowledge; but they don't deny that we have it, or that we can have it. Not so many philosophers, however, have gone so far as to say that we have no knowledge at all, or (to say something even stronger) that it is impossible to have knowledge.