The Immobility of God - Caput 4 "Quod Deus Est Immobilis" from "The Compendium of Theology" by Thomas Aquinas

Continuing from talking about the necessity of a first cause from chapter 3 in Aquinas' Compendium of Theology here Thomas gets into the connecting idea of God being immobile. 

To unpack Aquinas' reasoning here we must understand that "motus" refers to any type of change a thing can undergo, not just a change of place. But why does change occur at all? Change occurs when a thing's potential is actualized. But note that something cannot actualize itself in the same time/respect as it is in potency because of the Principle of Non Contradiction. Rather, a thing is only actualized by another more perfect thing which conveys its actuality. Even if this thing is another part of the same body, this principle holds absolutely. And if you take this to its limit case then you see that the first mover is immobile because it must, by definition, be complete actuality. 

It is helpful, though, to reference the 10 Categories of Aristotle here because these express the ways in which a thing can undergo change: substance, quantity, quality, relation, action, passion, posture, time, place, and possession. All of these modes of being can be summarized in two unifying words: actuality and potentiality. Actuality refers to the perfection or being that a thing currently possesses, while potentiality refers to the perfection or being that a thing does not currently possess but could acquire. The first mover requires taking all of these categories to their limit case of pure actuality, and thus is an immobile being as nothing could be added to it nor is it lacking in any way. 

Caput 4 - CHAPTER 4

Quod Deus est immobilis - THE IMMOBILITY OF GOD

Ex hoc apparet quod necesse est Deum moventem omnia, immobilem esse. Cum enim sit primum movens, si moveretur, necesse esset se ipsum vel a se ipso, vel ab alio moveri. Ab alio quidem moveri non potest: oporteret enim esse aliquid movens prius eo; quod est contra rationem primi moventis. A se ipso autem si movetur, hoc potest esse dupliciter. Vel quod secundum idem sit movens et motum; aut ita quod secundum aliquid sui sit movens, et secundum aliquid motum. Horum quidem primum esse non potest. Cum enim omne quod movetur, inquantum huiusmodi, sit in potentia; quod autem movet, sit in actu; si secundum idem esset movens et motum, oporteret quod secundum idem esset in potentia et in actu; quod est impossibile. Secundum etiam esse non potest. Si enim esset aliquod movens, et alterum motum, non esset ipsum secundum se primum movens, sed ratione suae partis quae movet. Quod autem est per se, prius est eo quod non est per se. Non potest igitur primum movens esse, si ratione suae partis hoc ei conveniat. Oportet igitur primum movens omnino immobile esse.

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.

Ex iis etiam quae moventur et movent, hoc ipsum considerari potest. Omnis enim motus videtur ab aliquo immobili procedere, quod scilicet non movetur secundum illam speciem motus; sicut videmus quod alterationes et generationes et corruptiones quae sunt in istis inferioribus, reducuntur sicut in primum movens in corpus caeleste, quod secundum hanc speciem motus non movetur, cum sit ingenerabile et incorruptibile et inalterabile. Illud ergo quod est primum principium omnis motus, oportet esse immobile omnino.

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.