The Tripartite Theory of Knowledge
There is a tradition that goes back as far as Plato that holds that three conditions must be satisfied in order for one to possess knowledge. This account, known as the tripartite theory of knowledge, analyses knowledge as justified true belief. The tripartite theory says that if you believe something, with justification, and it is true, then you know it; otherwise, you do not.
The first condition for knowledge, according to the tripartite theory, is belief. Unless one believes a thing, one cannot know it. Even if something is true, and one has excellent reasons for believing that it is true, one cannot know it without believing it.
The second condition for knowledge, according to the tripartite theory, is truth. If one knows a thing then it must be true. No matter how well justified or sincere a belief, if it is not true that it cannot constitute knowledge. If a long-held belief is discovered to be false, then one must concede that what was thought to be known was in fact not known. What is false cannot be known; knowledge must be knowledge of the truth.
The third condition for knowledge is justification. In order to know a thing, it is not enough to merely correctly believe it to be true; one must also have a good reason for doing so. Lucky guesses cannot constitute knowledge; we can only know what we have good reason to believe.
The tripartite theory of knowledge is intuitively very plausible. Since Edmund Gettier’s critique of it in the 60s, however, using thought-experiments now known as Gettier cases, it has been generally rejected. Nevertheless, it is still used as a working model by philosophers most of the time.