 While Avicenna has any number of arguments for the eternity of time and the world, his proof for the existence of time in terms of a possibility to traverse greater or lesser spatial magnitudes at a fixed velocity provides the basis for one of his central physical arguments for that thesis. The argument, which is a reductio ad absurdum has one assume that God created time, and so motion and moving bodies, at some first moment, x, in the finite past, for example, 10,000 years ago. Prior to x nothing but God existed, not even time. (Here let years be measured in solar rotations as measured by the apparent motion of our sun. Also allow that the present moment marks the completion of the 10,000 solar rotations, so that right now is the second terminal point for this motion.) Avicenna next observes that it is certainly possible that God could have made a time, and so motion and moving bodies, 20,000 solar rotations in duration, where this possible duration again reaches to the present moment as its second terminal point. In this second scenario, 10,000 years of the full 20,000 year duration would exactly correspond with the 10,000 years in the first scenario, namely, the period extending from the present moment back to x. The remaining duration of this possible 20,000-year motion would correspond with the possibility for 10,000 solar rotations, which now would terminate at x rather than beginning at x. (We can intuitively say that this remaining possibility is prior to or earlier than x, although Avicenna is careful not to beg any questions.) Now to say that there is a possibility for 10,000 solar rotations (prior to x) is just to say that (prior to x) there exist a possibility for a sun (or other body) to traverse a certain distance (that is, number of rotations) at a fixed speed. From Avicenna’s earlier proof for the existence of time, however, recall that the possibility for a moving body to traverse a given distance at a fixed speed refers to time. Consequently, since this possibility exists, time must exist. Yet in order for time to exist, as Avicenna argued, motion and moving bodies must exist. Therefore, concludes Avicenna, when nothing but God existed, time, motion and a moving body also would have necessarily existed, a blatant contradiction. It is the assumption that time is finite, claims Avicenna, that gives rise to the contradiction. Since this same argument applies to any finite duration of time, during which one may suppose the world to exist, Avicenna concludes that time, and so motion and moving bodies, must have existed infinitely into the past. (For the same reason  mutatis mutandis time, and so motion and moving bodies, also must exist infinitely into the future.)