Epistemology is the philosophical study of knowledge. But what is knowledge? To know something is to be acquainted with, or to have an understanding of, some fact or set of facts. Knowledge is typically understood in philosophy to be a type of belief, a mental attitude towards some proposition. Obviously, however, not all beliefs we have count as knowledge. In particular, we generally do not think it is possible to know something that is not actually true. But knowledge is still more than just a true belief, because we don’t typically think that someone knows something if they happen to believe it by pure luck. For example, someone who buys a lottery ticket every week might always overconfidently believe that they will win, and one week they may get lucky and actually really win. We would not say, however, that this person knew they were going to win, even though they correctly believed it. Philosophers have thus advanced a third condition, justification. Thus, in order to count as knowledge, a belief must be both true and justified. This would rule out the case of the lottery winner, since they had no justification for believing that they would win. What exactly counts as a justification has, needless to say, been subject to considerable dispute. There have also been attempts to discredit the traditional justified true belief account of knowledge by developing cases where a person has a justified true belief, but not knowledge. In spite of these disputes and complexities, the concept of knowledge is foundational to rationality, reasoning, and philosophy.