What then is time?
If no one asks me, I know; if I want to explain it to someone who does ask me, I do not know. Yet I state confidently that I know this: if nothing were passing away, there would be no past time, and if nothing were coming, there would be no future time, and if nothing existed, there would be no present time. How then can these two kinds of time, the past and the future be, when the past no longer is and the future as yet does not be? But if the present were always present, and would not pass into the past, it would no longer be time but eternity. Therefore, if the present, so as to be time, must be so constituted that it passes into the past, how can we say that it is, since the cause of its being is the fact that it will cease to be? Does it not follow that we can truly say that it is time, only because it tends towards non-being?
Mystery of time: Time past cannot be long, since it doesn’t exit, and the future does not yet exist, so only the present can have any length; yet the present can be divided into minute parts.
See how the present time, which alone we found worthy to be called long, is contracted … that one hour itself goes on in fleeting moments; whatever part of it has flown away is past, whatever remains is future. If any point of time is conceived that can no longer be divided into even the most minute parts of a moment, that alone is what may be called the present. It flies with such speed from the future into the past that it cannot be extended by even a trifling amount. For if it is extended, it is divided into past and future. The present has no space.
Time is measured as it is passing; but when it passes, it no longer exists.
But there is a past and future; but what are these, secret places where time goes and comes from?
They only exist in the present,
It is now plain and clear that neither past nor future are existent, and that it is not properly stated that there are three times, past, present, and future. But perhaps it might properly be said that there are three times, the present of things past, the present of things present, and the present of things future. These three are in the soul, but elsewhere I do not see them: the present of things past is in memory; the present of things present is in intuition; the present of things future is in expectation.
Source: Confessions, Book 11