Since then is not without parts, it must be divisible, and the changing thing must have changed in every part of it (for if it has changed in neither of the two parts into which is divided, it has not changed in the whole either; if, on the other hand, it is changing in both parts, it is likewise changing in the whole; and if, again, it has changed in one of the two parts, the whole is not the primary time in which it has changed: it must therefore have changed in every part).