Analyzing the concept of knowledge has commonly been taken to be one of the central tasks, if not the central task, of epistemology. There are several different uses of “know” (as in “I know John,” “I know how to drive,” and “I know your phone number”), but the sense that epistemologists have focused on is the sense that refers to propositional knowledge, or factual knowledge. This is the sense involved in “I know that 2 + 2 = 4” and “Does John know that the game has been delayed?”