Augustine's exploration of the nature of time in Book XI is a fascinating exercise. He notes the difficulty of it in this famous line: "What, then, is time? If no one asks me, I know; if I want to explain it to someone, I do not know." He sees the paradox of talking about time while remaining in time, a paradox similar to using the mind to discuss the mind. Augustine must talk about time in order to justify his time-bound autobiographical account, and because it leads to a discussion of eternity and God.