In what way does this not negate polytheism?
Secondly, this is proved from the infinity of His perfection. For it was shown above (I:4:2) that God comprehends in Himself the whole perfection of being. If then many gods existed, they would necessarily differ from each other. Something therefore would belong to one which did not belong to another. And if this were a privation, one of them would not be absolutely perfect; but if a perfection, one of them would be without it. So it is impossible for many gods to exist. Hence also the ancient philosophers, constrained as it were by truth, when they asserted an infinite principle, asserted likewise that there was only one such principle.
Aquinas never really interacts with polytheism as such. What I mean is that in every case he objects to “polytheism”, he does so by holding “Gods” to a standard that
only applies to creatures, thereby failing to treat of those who would
truly be Gods, and thus of polytheism as such. This was hard for me to swallow when I was a Thomist, but once you see it, it’s as clear as day.
The objection you cite, which has been given by many Christian authors throughout Church history, provides a particularly good illustration of what I’m talking about.
There’s a difference between individuation and differentiation: individuation precedes differentiation, as differentiation is just the contrasting of one thing which is already individuated from another. To put things linguistically to illustrate: to be individuated is to be the subject without any predicates, whereas to be differentiated is for the predicates of one subject to be contrasted with those of another. Strictly speaking, individuality is ineffable, since a subject without any predicates has no predicates by which to be described.
So,
creatures have “predicates”, as it were, and we differentiate one creature from another by contrasting their “predicates.” But, that which is God is utterly simple, and so has no “predicates” whatsoever, let alone any to be contrasted with from others. As such, to demand that one God be differentiated from another by virtue of contrasting “predicates” is not to treat them like they’re
really “Gods”, but rather to hold them to a standard that only applies to creatures: it is to fundamentally misunderstand what polytheism would involve, and so to straw-man it. If we are to
genuinely treat of a plurality of “Gods”, then we’ll take seriously the fact that
each would be utterly simple. Since Gods would not have any “predicates” by which to differ from one another, each would have to differ from another simply by virtue of his or her peculiar and ineffable individuality: they would each be non-identical subjects.
Aquinas demands that “Gods” differ from one another by virtue of something
other than their individuality – namely, by
having this perfection, or that one. In so doing, he attacks a straw-man, and leaves polytheism as such untouched.