Augustine of Hippo, Confessions XI.21: How Time May Be Measured

27. I have just now said, then, that we measure times as they pass, that we may be able to say that this time is twice as much as that one, or that this is only as much as that, and so of any other of the parts of time which we are able to tell by measuring. Wherefore, as I said, we measure times as they pass. And if any one should ask me, “Whence dost thou know?” I can answer, “I know, because we measure; nor can we measure things that are not; and things past and future are not.” But how do we measure present time, since it hath not space? It is measured while it passeth; but when it shall have passed, it is not measured; for there will not be aught that can be measured. But whence, in what way, and whither doth it pass while it is being measured? Whence, but from the future? Which way, save through the present? Whither, but into the past? From that, therefore, which as yet is not, through that which hath no space, into that which now is not. But what do we measure, unless time in some space? For we say not single, and double, and triple, and equal, or in any other way in which we speak of time, unless with respect to the spaces of times. In what space, then, do we measure passing time? Is it in the future, whence it passeth over? But what yet we measure not, is not. Or is it in the present, by which it passeth? But no space, we do not measure. Or in the past, whither it passeth? But that which is not now, we measure not.