Ch. 15 - "The Unicity of God" from "The Compendium of Theology" by Thomas Aquinas

Chapter 15 - THE UNICITY OF GOD

Chapter fifteen continues Aquinas' treatment of God in the Compendium of Theology, and thus will draw full force if the reader is familiar with what he has established in the first fourteen chapters. In Chapter fifteen he is focusing on the "unicity of God", or that God is only one. Just to define a few terms before unpacking what Aquinas is saying, we will need to know what "equivocal" and "univocal" mean, as well as the term "individuated". Equivocal refers to two words which said with the same word/sound, but have different meanings, such as the word "spring" can mean a source of flowing water, or a metal object in one's bed. Univocal refers to two things which are spoken of in the same way, "with one voice," so to speak. When I speak of my cat and your cat, I am speaking of them univocally, as they are both cats. 

Thus, Aquinas says that if we use the word "God" in an equivocal sense, it is just plain ridiculous because then we can give the word God any meaning whatsoever. My kitchen table is god, for example. So it doesn't carry any weight to say there are two gods, yet mean the term equivocally. If we say there are two gods in an univocal sense, then they must be two individuals of the same genus and species, but Aquinas has already established that God cannot be contained in any genus or species, as he transcends them all. Thus, we cannot speak univocally of two gods either. 

He explains more, that when it comes to essences being individuated by real existence the universal essence, such as dog-ness, cannot be more than one since it contains all of itself in that one essence. (Caveat, this does not happen to any material beings, but does apply to God.) Likewise, it is impossible for an individuated expression of that essence, such as a man, to include all of the universal man-ness in himself. And yet God is his essence, not some instantiation of it. Therefore, God can only be one, not many. 

The third paragraph of chapter fifteen makes this point with an example. "A form can be multiplied in two ways: first, by specific differences, as in the case of a generic form;.in this way color is differentiated into the various species of color; secondly, by the subjects in which it inheres, for example, whiteness. Therefore any form incapable of being multiplied by specific differences cannot be multiplied at all, if it is a form that does not exist in a subject. Thus whiteness, if it were to subsist without a subject, would not be more than one. But the divine essence is very existence, ipsum esse, which does not admit of specific differences, as we have shown. Since, therefore, the divine existence is a quasi-form subsisting by itself, seeing that God is His existence, the divine essence cannot be more than one. Accordingly a plurality of gods is impossible."

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https://isidore.co/aquinas/english/Compendium.htm#15