But in what sense can we say that those two times, the past and the future, exist, when the past no longer is and the future is not yet? Yet if the present were always present and did not go by into the past, it would not be time at all, but eternity. If, therefore, the present (if it is to be time at all) only comes into existence because it is in transition toward the past, how can we say that even the present is? For the cause of its being is that it shall cease to be.
— Augustine, Confessions (quoted here)