Thank you for subscribing to Annotated Thomist...check back each day for a new section of St. Thomas' corpus, annotated and summarized. (FREE TRIAL FOR NEW SUBSCRIBERS!!!)
If you need more personalized help reading the Summa, I am available for 1-on-1 sessions, here.
To begin our inquiry into this thorny question, we must think a little about how language is used. Terms can be used in two ways, either, 1. Concretely, or 2. Abstractly.
In the first, we signify something as subsisting in some individual. Thus, we say this man is "white," not whiteness.
In the second, we signify a pure form. Thus, we speak of the universal "whiteness," not white.
Yet, God is BOTH subsisting and a pure form (as we have proven above). Thus, we ought to carefully move back and forth between the the concrete "God" and the abstract "Deity." This question asks the question of whether "God is his Deity," i.e., whether we signify something really distinct when we speak of "God" and when we speak of "Deity," as we would in the case of "man" and his "humanity."