Why, as existence, of course...
You said: Aquinas would identify the apprehension of existence and essence (which includes composition [the substantial unity of this particular thing] and division [this thing is not that thing]) as the "first act of the mind."
Aquinas would be wrong.
How can he be wrong if he agrees with you?
Aquinas would be wrong.
How can he be wrong if he agrees with you? Because he does not agree with me at all.
The first disagreement is minor, I would not call "apprehension" and act of the "mind" at all, because when talking about the mind I would be referring to the conceptual level of consciousness, not the perceptual, which is what a simple apprehension (or percept) is. Percepts do not include any cognitive elements such as "composition" or "unity" or "divisions." Aquinas confuses perception and conception, which is what I originally pointed out, and this confusion makes his epistemology hopeless.
Hank