The argument from motion, again, followed by some general remarks:
The first and plainest is the method that proceeds from the point of view of motion. It is certain and in accord with experience, that things on earth undergo change. Now, everything that is moved is moved by something; nothing, indeed, is changed, except it is changed to something which it is in potentiality.
Moreover, anything moves in accordance with something actually existing; change itself, is nothing else than to bring forth something from potentiality into actuality. Now, nothing can be brought from potentiality to actual existence except through something actually existing: thus heat in action, as fire, makes fire-wood, which is hot in potentiality, to be hot actually, and through this process, changes itself. The same thing cannot at the same time be actually and potentially the same thing, but only in regard to different things. What is actually hot cannot be at the same time potentially hot, but it is possible for it at the same time to be potentially cold. It is impossible, then, that anything should be both mover and the thing moved, in regard to the same thing and in the same way, or that it should move itself.
Everything, therefore, is moved by something else. If, then, that by which it is moved, is also moved, this must be moved by something still different, and this, again, by something else. But this process cannot go on to infinity because there would not be any first mover, nor, because of this fact, anything else in motion, as the succeeding things would not move except because of what is moved by the first mover, just as a stick is not moved except through what is moved from the hand.
Therefore it is necessary to go back to some first mover, which is itself moved by nothing—and this all men know as God.
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“Motion” or change in the argument means the reduction of a thing from the state of potentiality to one of act. Accordingly, everything which is moved must be moved by another. In order to avoid an infinite regress in the chain of movers, the existence of an unmoved mover must be acknowledged.
In regard to an “infinite regress”, there appears to be no logical reason known why there cannot be infinite series of events or an infinite series of causes and effects stretching back into the past. Such an infinite series can be conceived of in which any given member or event, X , depended on a preceding event, Y, but once it exists no longer depends here and now for its existence on the prior event. Such a lineal or “horizontal”, and infinite series stretching back into the past would still require an ultimate explanation of its motion, even though the series is infinite.
That is, there is a “vertical” hierarchy in which the lower members or events depend here and now on the present causality of the members or events above it. This vertical hierarchy is one that cannot have an infinite regress of causal dependencies. Perhaps one can imagine here and now a tree whose existence is dependent on many factors in the environment, light, air, water, etc. Those factors depend on the earth, which depends on the sun, and so on. This here and now vertical regress of movers and things moved must terminate in an unmoved mover. Eliminate the unmoved mover and you will have eliminated the entire chain of motion and failed to provide any explanation for motion or change.
