Page:Our knowledge of the external world.djvu/193

, the three men B, B', B  in one row, and the three men C, C', C  in the other row, are respectively opposite to A, A', and A ''. At the very next moment, each row has moved on, and now B and C '' are opposite A'. Thus B and C '' are opposite each other. When, then, did B pass C'? It must have been somewhere between the two moments which we supposed consecutive, and therefore the two moments cannot really have been consecutive. It follows that there must be other moments between any two given moments, and therefore that there must be an infinite number of moments in any given interval of time.

The above difficulty, that B must have passed C' at some time between two consecutive moments, is a genuine one, but is not precisely the difficulty raised by Zeno. What Zeno professes to prove is that “half of a given time is equal to double that time.” The most intelligible explanation of the argument known to me is that of Gaye. Since, however, his explanation is not easy to set forth shortly, I will re-state what seems to me to be the logical essence of Zeno’s contention. If we suppose that time consists of a series of consecutive instants, and that motion consists in passing through a series of consecutive points, then the fastest possible motion is one which, at each instant, is at a point consecutive to that at which it was at the previous instant. Any slower motion must be one which has intervals of rest interspersed, and any faster motion must wholly omit some points. All this is evident from the fact that we cannot have more than one event for each instant. But now, in the case of our A’s and B’s and C’s, B is opposite a fresh A every instant, and therefore the number of A’s passed gives the number of instants since the beginning of the motion. But during the motion B has passed twice as