Augustine on Time
“At no time then had You not made anything, for time itself You made. And no time is co-eternal with You, for You stand changeless; whereas if time stood changeless, it would not be time. What then is time? Is there any short and easy answer to that? Who can put the answer into words or even see it in his mind? Yet what commoner or more familiar word do we use in speech than time? Obviously when we use it, we know what we mean, just as when we hear another use it, we know what he means.
What then is time? If no one asks me, I know; if I want to explain it to a questioner, I do not know. But at any rate this much I dare affirm I know: that if nothing passed there would be no past time; if nothing were approaching, there would be no future time; if nothing were, there would be no present time. But the two times, past and future, how can they be, since the past is no more and the future is not yet? On the other hand, if the present were always present and never flowed away into the past, it would not be time at all, but eternity. But if the present is only time, because it flows away into the past, how can we say that it is? For it is, only because it will cease to be. Thus we can affirm that time is only in that it tends towards not-being.”
— Confessions by Saint Augustine, Translated by F. J. Sheed. Introduction by Peter Brown, Notes by Michael Foley, Book XI, XIV page 242.