Enneads III.7: Of Time and Eternity
B. Time
The Opinions of the Philosophers About Time Must Be Studied
If those blessed ancient philosophers had not already uttered their views about time, we would only need to add to the idea of eternity what we have to say of the idea of time, and to set forth our opinion on the subject, trying to make it correspond with the already expressed notion of eternity. But we now must examine the most reasonable opinions that have been advanced about time, and observe how far our own opinion may conform thereto.
TIME CONSIDERED EITHER AS MOTION; AS SOMETHING MOVABLE; OR SOMETHING OF MOTION.
To begin with, we may divide the generally accepted opinions about time into three classes: time as movement, as something movable, or as some part of movement. It would be too contrary to the notion of time to try to define it as rest, as being at rest, or as some part of rest; for time is incompatible with identity (and consequently with rest, and with what is at rest). Those who consider time as movement, claim that it is either any kind of movement, or the movement of the universe. Those who consider it as something movable are thinking of the sphere of the universe; while those who consider time as some part of movement consider it either as the interval of movement, or as its measure, or as some consequence of movement in general, or regular movement.