  Aristotle argues at the opening of Physics bk. 8 that motion and change in the universe can have no beginning, because the occurrence of change presupposes a previous process of change. With this argument Aristotle can establish an eternal chain of motions and refute those who hold that there could have been a previous stationary state of the universe. Such an eternal chain, Aristotle argues, needs to rely on a cause which guarantees its persistence: if each of the constitutive processes in the causally connected web were of finite duration, for every one of them it can be the case that it is not present in the world, indeed, at some later time it will not be present any longer. But then the whole causally connected series of events, Aristotle claims, would also be  contingent.[34]  Hence Aristotle postulates that the processes of the universe depend on an eternal motion (or on several eternal motions), the eternal revolution of the heavenly spheres, which in turn is dependent on one or several unmoved movers (Physics 8.6,  258b26–259a9).[35]