In everyday discourse in the English language, the word “knowledge” means justified true belief. According to this definition, for a person to have knowledge of the truth of a given proposition, the proposition in question must be true, the person considering the proposition must believe it to be true, and the person in question must be justified in believing it to be true. Some thinkers, most famously contemporary American philosopher Edmund Gettier, have called into question whether knowledge means justified true belief on the grounds it is possible to have justified true belief in a proposition even if the purported justifications for the belief are false.[1]

For example, suppose a person walking down a dimly lit path at night sees a coiled rope in the path, believes the rope to be a snake, and thus believes there is a snake in the path. Also suppose that ten meters up the path from the rope there is a coiled snake that the person walking does not see. In this situation, the person has justified true belief because it is true that there is a snake in the path, the person believes there is a snake in the path, and the person is justified in this belief because the rope in the path looks like a snake. However, the person’s justification for believing there is a snake in the path is obviously false. In this situation, while all three criteria of justified true belief are present, this is clearly insufficient for claiming that the person has knowledge in the ordinary sense of the word. Another element is necessary. That element is true justification for the belief. Thus, knowledge requires not only justified true belief, but that the justification for the belief also be true. Put another way, if a person’s justification for a belief is false, there is no knowledge, even if the belief happens to be true for other reasons. Thus, knowledge must mean truly justified true belief. While this is an awkward turn of phrase, it must suffice because no more elegant phrasing is possible without compromising the meaning.

[1] Edmund Gettier, “Is Justified True Belief Knowledge?” Analysis 23(6): 121-123 (1963).