The Six Books of Proclus on the Theology of Plato, Volume 2

For since it is measured by time, the motion of it also participates of measure and bound, and proceeds according to number. But because it is always moved, and this always is not[315] eternal, but temporal, it is necessary that it should use periods. For motion indeed is a mutation from some things to others. But beings are terminated by multitude and magnitude. These, however, being bounded, there can neither be a mutation to infinity according to a right line, nor can that which is always moved, make its transitions finitely. Hence, that which is always moved, will proceed from the same to the same, and thus will proceed periodically.