The Thomist manual tradition divided metaphysics into the study of finite and infinite being. The governing principle here is the primacy of esse or the act of existence, which is either recieved (finite being) or unrecieved (infinite). Another division hinted at by some of the manuals might be better: an analysis of the question “what exists”; that is, an analysis of the subject that we say exists. Here the division is between essence (which is what exists so far as existence must appropriate something outside of itself in order to exercise its act, and is therefore what exists in the mode of the perfected) and God himself (who is “what exists” when the question is answered per se in the first mode of the per se.)
The division might be between essence, which is what exists in the second mode of the per se, and God, who exists in the first mode of the per se. Essence can enter into the account of existence (in the mode that matter enters into a definition), whereas saying that God exists is like saying that a circle is equidistant from the center.