(02-14-2013, 01:28 PM)Scotus Wrote: [ -> ]I am planning to give a talk in my local parish on the proofs for the existence of God. I was planning to spend the bulk of the time on the First Way of St Thomas Aquinas i.e. the Argument from Motion. However, as I have been working out how to explain each step of the proof there seems (to me at least) in the assertion that the First Mover must be Pure Act. Certainly, it is clear that this Unmoved Mover is not moved by any other (as we would carry on with our infinite regress), but it does seem to me that there is a jump from proving that there is an Unmoved Mover to stating that this Mover is immovable. In the order of causes this being must be in act in such a way that it is has not been reduced to that act by any other being. But how can we prove that there is no potency in this being, even though these potencies might never be reduced to act?
If we can prove that this Unmoved Mover is Pure Act then it seems that we have failed to prove the existence of God by the evidence of motion in the world.
This might help from his Conpendium,
"We clearly infer from this that God, who moves all things, must Himself be immovable. If He, being the first mover, were Himself moved, He would have to be moved either by Himself or by another. He cannot be moved by another, for then there would have to be some mover prior to Him, which is against the very idea of a first mover. If He is moved by Himself, this can be conceived in two ways: either that He is mover and moved according to the same respect, or that He is a mover according to one aspect of Him and is moved according to another aspect. The first of these alternatives is ruled out. For everything that is moved is, to that extent, in potency, and whatever moves is in act. Therefore if God is both mover and moved according to the same respect, He has to be in potency and in act according to the same respect, which is impossible. The second alternative is likewise out of the question. If one part were moving and another were moved, there would be no first mover Himself as such, but only by reason of that part of Him which moves. But what is per se is prior to that which is not per se. Hence there cannot be a first mover at all, if this perfection is attributed to a being by reason of a part of that being. Accordingly the first mover must be altogether immovable.
Among things that are moved and that also move, the following may also be considered. All motion is observed to proceed from something immobile, that is, from something that is not moved according to the particular species of motion in question, Thus we see that alterations and generations and corruptions occurring in lower bodies are reduced, as to their first mover, to a heavenly body that is not moved according to this species of motion, since it is incapable of being generated, and is incorruptible and unalterable. Therefore the first principle of all motion must be absolutely immobile.
... God’s essence cannot be other than His existence. In any being whose essence is distinct from its existence, what it is must be distinct from that whereby it is. For in virtue of a thing’s existence we say that it is, and in virtue of its essence we say what it is. This is why a definition that signifies an essence manifests what a thing is. In God, however, there is no distinction between what He is and that whereby He is, since there is no composition in Him, as has been shown. Therefore God’s essence is nothing else than His existence.
Likewise, we have proved that God is pure act without any admixture of potentiality. Accordingly His essence must be the ultimate act in Him; for any act that has a bearing on the ultimate act, is in potency to that ultimate act. But the ultimate act is existence itself, ipsum esse. For, since all motion is an issuing forth from potency to act, the ultimate act must be that toward which all motion tends; and since natural motion tends to what is naturally desired, the ultimate act must be that which all desire. This is existence. Consequently the divine essence, which is pure and ultimate act, must be existence itself, ipsum esse."
Also to consider, nothing can be both potential and actual in the same respect at the same time, what is actually hot is not at the same time potentially hot but potentially cold. Hence it is impossible for anything to be both, in the same respect and time (whether it be something part of an essential ordered series or an accidental ordered series), both that which is moved and moving.