Again, if a thing is capable of any particular motion, it is also capable of the corresponding contrary motion or the corresponding coming to rest, and a thing that is capable of becoming is also capable of perishing: consequently, what is in process of becoming in process of becoming is in process of perishing at the very moment when it is in process of becoming in process of becoming; since it cannot be in process of perishing when it is just beginning to become or after it has ceased to become; for that which is in process of perishing must be in existence.