27. I have said, then, that we measure periods of time as they pass so that we can say that this time is twice as long as that one or that this is just as long as that, and so on for the other fractions of time which we can count by measuring.
So, then, as I was saying, we measure periods of time as they pass. And if anyone asks me, "How do you know this?", I can answer: "I know because we measure. We could not measure things that do not exist, and things past and future do not exist." But how do we measure present time since it has no extension? It is measured while it passes, but when it has passed it is not measured; for then there is nothing that could be measured. But whence, and how, and whither does it pass while it is being measured? Whence, but from the future? Which way, save through the present? Whither, but into the past? Therefore, from what is not yet, through what has no length, it passes into what is now no longer. But what do we measure, unless it is a time of some length? For we cannot speak of single, and double, and triple, and equal, and all the other ways in which we speak of time, except in terms of the length of the periods of time. But in what "length," then, do we measure passing time? Is it in the future, from which it passes over? But what does not yet exist cannot be measured. Or, is it in the present, through which it passes? But what has no length we cannot measure. Or is it in the past into which it passes? But what is no longer we cannot measure.