Time and Motion

Aristotle begins his analysis of time by noting its puzzling nature. It seems to consist of a past that no longer exists and a future that does not yet exist, connected by an instant — the "now" — that is not itself a stretch of time. Yet time clearly exists: we experience it, measure it, and are subject to it. His solution is to tie time tightly to motion. Time is not the motion itself — motions can be fast or slow — but the measure of motion, the counting of how many "before-and-after" intervals a change involves.

The now (to nun) plays a peculiar double role. As a boundary, it separates the past from the future — it is the edge of what was and the beginning of what will be. But boundaries are not parts: the point at the end of a line is not a segment of the line. As a connector, the now is what gives time its continuity — the same now that ends the past begins the future, just as the middle of a journey is both the end of the first half and the start of the second. Time is one continuous whole because its nows, though different in their before-and-after relations, are the same in substrate.

One of Aristotle's most striking observations is that without a counting soul, time might not exist. Motion and change would still occur — bodies would still move, things would still alter — but the before-and-after would not be numerated, and without numeration there is no number, and time is the number of motion. This anticipates the modern debate about whether time is observer-dependent. Aristotle does not conclude that time is subjective — it has its ground in the objective order of motion — but he insists that actual temporal measurement requires a mind to do the counting.

Time is analysed in Physics IV.10–14. Aristotle's definition — "the number of motion with respect to before and after" — became the standard account in ancient and medieval philosophy. Augustine's famous meditation on time in the Confessions (Book XI) is in part a response to Aristotle, shifting the analysis from motion to the mind's distension.