SCG I, 42

Chapter 42: THAT GOD IS ONE [Quod Deus unus est]

… [2] For it is not possible that there be two highest goods, since that which is said by superabundance is found in only one being. But God, as we have shown, is the highest good. God is, therefore, one.

[3] Again, it has been shown that God is absolutely perfect, lacking no perfection. If, then, there are many gods, there must be many such perfect beings. But this is impossible. For, if none of these perfect beings lacks some perfection…, nothing will be given in which to distinguish the perfect beings from one another. It is impossible, therefore, that there be many gods.

[4] Again, that which is accomplished adequately through one supposition is better done through one than through many. But the order of things is the best it can be, since the power of the first cause does not fail the potency in things for perfection. Now, all things are sufficiently fulfilled by a reduction to one first principle. There is, therefore, no need to posit many principles.

Whiffs of "Ockham's razor" decades before Ockham.

[5] Moreover, it is impossible that there be one continuous and regular motion from many movers. For, if they move together, none of them is a perfect mover, but all together rather take the place of one perfect mover. This is not befitting in the first mover, for the perfect is prior to the imperfect. If, however, they do not move together, each of them at times moves and at times does not. It follows from this that motion is neither continuous nor regular. … But, as the philosophers have proved, the first motion is one and continuous. Therefore, its first mover must be one.

[6] Furthermore, a corporeal substance is ordered to a spiritual substance as to its good. For there is in the spiritual substance a fuller goodness to which the corporeal substance seeks to liken itself, since whatever exists desires the best so far as this is possible. … outside the spiritual substance that is the end of the first motion, there is none that is not reduced to it. But this is what we understand by the name of God. Hence, there is only one God.

[7] Among all the things that are ordered to one another, furthermore, their order to one another is for the sake of their order to something one; just as the order of the parts of an army among themselves is for the sake of the order of the whole army to its general. For that some diverse things should be united by some relationship cannot come about from their own natures as diverse things, since on this basis they would rather be distinguished from one another. Nor can this unity come from diverse ordering causes, because they could not possibly intend one order in so far as among themselves they are diverse. Thus, either the order of many to one another is accidental, or we must reduce it to some one first ordering cause that orders all other things to the end it intends. Now, we find that all the parts of this world are ordered to one another according as some things help some other things. … Nor is this something accidental, since it takes place always or for the most part. Therefore, this whole world has only one ordering cause and governor. But there is no other world beyond this one. Hence, there is only one governor for all things, whom we call God.

This is parallel to the fifth way in ST I.2.3, which is, incidentally, my favorite way:

"We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."

[8] Then, too, if there are two beings of which both are necessary beings, they must agree in the notion of the necessity of being. Hence, they must be distinguished by something added either to one of them only, or to both. This means that one or both of them must be composite. Now, as we have shown, no composite being is through itself a necessary being. It is impossible therefore that there be many beings of which each is a necessary being. Hence, neither can there be many gods.

[9] Furthermore, given two gods that are posited as agreeing in the necessity of being, either that in which they differ is in some way required for the completion of their necessity of being, or it is not. If it is not, then it is something accidental…. …unless that something else existed, this accident would not exist; and unless this accident existed, the aforesaid distinction would not exist. Therefore, unless that something else existed, these two supposed necessary beings would not be two but one. Therefore, the proper being of each depends on the other, and thus neither of them is through itself a necessary being.

[10] If, however, that in which they are distinguished is required to complete the necessity of their being, either this will be because it is included in the nature of this necessity of being, as animate is included in the definition of animal, or this will be because their necessity of being is specified by it, as animal is completed by rational. … A difference specifying a genus does not complete the nature of the genus, but rather through it the genus comes to be in act. For the nature of animal is complete before the addition of rational. Rather, the fact is that there cannot be an animal in act that is not rational or irrational. Thus, therefore, something completes the necessity of being as to being in act, and not as to the notion of the necessity of being. This is impossible on two counts. First, because the quiddity of a necessary being is its being, as was proved above. Second, because, were it true, the necessary being would acquire being through something else, which is impossible. …

[12] What is more, if there are two gods, either the name God is predicated of both univocally, or equivocally. … But if it be used univocally, it must be predicated of both according to one notion, which means that, in notion, there must be in both one nature. … If according to one, there will not be two gods, but only one, since there cannot be one being for two things that are substantially distinguished. If each has its own being, therefore in neither being will the quiddity be its being. Yet this must be posited in God….

[13] Again, nothing that belongs to this designated thing as such can belong to another, for the singularity of some thing belongs to none other than to that singular thing. But its necessity of being belongs to the necessary being so far as it is this designated being. Therefore, it cannot belong to another, and therefore there cannot be several beings of which each is a necessary being. …

[14] The proof of the minor. If the necessary being is not this designated being as a necessary being, the designation of its being is not necessary through itself but depends on another. But so far as each thing is in act it is distinct from all other things; this is to be this designated thing [hoc aliud res designata? So much for my stab at the Latin, which in fact is: quod est esse hoc signatum.]. Therefore, the necessary being depends on another to be in act; which is against the nature of the necessary being. Therefore, the necessary being must be necessary according as it is this designated being. …

[17] Then, too, the proper being of each thing is only one. But God is His being, as we have shown. There can, therefore, be only one God.

[18] Moreover, a thing has being in the manner it possesses unity. Hence, each thing struggles as much as it can against any division of itself, lest thereby it tend to nonbeing. But the divine nature has being most powerfully. There is therefore, in it the greatest unity, and hence no plurality is in any way distinguished within it.

Cf. SCG I.41.4

[19] Furthermore, we notice in each genus that multitude proceeds from some unity. This is why in every genus there is found a prime member that is the measure of all the things found in that genus. In whatever things, therefore, we find that there is an agreement in one respect, it is necessary that this depend upon one source. But all things agree in being. There must, therefore, be only one being that is the source of all things. This is God. …

Cf. SCG I.39.7. It is also parallel to the fourth way in ST I.2.3:

"Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God."

[22] Now by this truth are refuted those Gentiles who accepted a multitude of gods. However, many of them said that there was one highest God, by whom all the others whom they named gods were according to them caused. For they attributed the name of divinity to all everlasting substances, and this especially because of their wisdom and felicity and the rulership of things. …

[23] Hence, it is mainly the Manicheans who seem opposed to this truth, in that they posit two first principles of which one is not the cause of the other.

[24] The Arians likewise attacked this truth by their errors, in confessing that the Father and the Son are not one but several gods; although the authority of Scripture forces me to believe that the Son is true God.