"Compendium of Theology" Ch. 3 - "The Existence of God" by Thomas Aquinas

A Few Thoughts 
The Compendium of Theology is a summarized version of his work that he explains in more detail in other places. And so here Thomas starts with a summary of the essence of the metaphysical ways to God. At their core, they are based on the universal principle of causality. This principle requires that all reasoning be led to its limit case, i.e. a cause which is perfectly existing of itself and can act as a first cause to all instrumental or secondary causes/beings. There is no other conclusion to which the principle of causality can lead without denying that principle. And the problem is that if one denies the principle of causality, then they have in the very act affirmed it because it too must apply to our thoughts which are based on reality. So if reality is not coherent in terms of causality, then neither are our thoughts or arguments we make against it. The true skeptic must be like Aristotle's vegetable, quiet and unmoving. 

Another key point from the Latin in this chapter is to focus on the word "esse". "Esse" is the infinitive form of the verb "to be" in Latin and is the basis of our attempts in language to understand God. This is because esse is the broadest way of speaking about something. Being is the broadest category of existence or actuality. And so, like in Exodus, when God says that his name is "I AM" it shows that God is perfect being, not some aspect or limitation of being like the pagan gods or any created thing. Zeus is the embodiment of power, Aphrodite of love, Ares of war ... and so on. But these are all limitations and not befitting of the prime mover. Thus, God can only be spoken about as "I AM", as he has no limits to his being. 

The Text 

Caput 3 - CHAPTER 3

Quod Deus sit - THE EXISTENCE OF GOD

Circa essentiae quidem divinae unitatem primo quidem credendum est Deum esse; quod ratione conspicuum est. Videmus enim omnia quae moventur, ab aliis moveri: inferiora quidem per superiora, sicut elementa per corpora caelestia; et in elementis quod fortius est, movet id quod debilius est; et in corporibus etiam caelestibus inferiora a superioribus aguntur. Hoc autem in infinitum procedere impossibile est. Cum enim omne quod movetur ab aliquo, sit quasi instrumentum quoddam primi moventis; si primum movens non sit, quaecumque movent, instrumenta erunt. Oportet autem, si in infinitum procedatur in moventibus et motis, primum movens non esse. Igitur omnia infinita moventia et mota erunt instrumenta. Ridiculum est autem etiam apud indoctos, ponere instrumenta moveri non ab aliquo principali agente: simile enim est hoc ac si aliquis circa constitutionem arcae vel lecti ponat serram vel securim absque carpentario operante. Oportet igitur primum movens esse, quod sit omnibus supremum; et hoc dicimus Deum.

Regarding the unity of the divine essence, we must first believe that God exists. This is a truth clearly known by reason. We observe that all things that move are moved by other things, the lower by the higher. The elements are moved by heavenly bodies; and among the elements themselves, the stronger moves the weaker; and even among the heavenly bodies, the lower are set in motion by the higher. This process cannot be traced back into infinity. For everything that is moved by another is a sort of instrument of the first mover. Therefore, if a first mover is lacking, all things that move will be instruments. But if the series of movers and things moved is infinite, there can be no first mover. In such a case, these infinitely many movers and things moved will all be instruments. But even the unlearned perceive how ridiculous it is to suppose that instruments are moved, unless they are set in motion by some principal agent. This would be like fancying that, when a chest or a bed is being built, the saw or the hatchet performs its functions without the carpenter. Accordingly there must be a first mover that is above all the the rest; and this being we call God.

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.