Since, however, every thing which changes, changes in time, and a thing is said to change in time, both as in that which is first, and as with reference to something else; as for instance, in a year because it changes in a day; — this being the case, it is necessary that a thing should change in every part of that time in which that which first changes, changes. But this is manifest from definition: for we thus denominate what is first. It is also evident, from the following considerations: for let that in which a thing that is first moved, is moved be X R; and let it be divided in K; for all time is divisible; in the time X K, therefore, it will either be moved, or not be moved. And again, in a similar manner in the time K R. If, therefore, it is moved in neither, it will be at rest in the whole time; since it is impossible for that to be moved, whicli is moved in no part of this. But if it is alone moved in the other, it will not have been first moved in X R; for motion subsists with reference to another. It is necessary, therefore, that it should be moved in every part of X R.
