We’re entering an arcane patch, a thicket of argument to prove a technical point about God’s intelligence. I’ll skip around some of this material, but not all of it. We need it to get to the punchline, which is coming in 15 chapters, that God is Truth. From Truth we move to Good and Evil. So this thicket is well worth punching through, though I admit that some attentions will flag.
 FROM what has been proved above it is made evident that the divine intellect understands by no other intelligible species but the divine essence.
Notes Think of “intelligible species” as the how of how God thinks. What Aquinas is doing is showing, or rather illuminating, a point he already proved, that God is simple (in the meaning of that technical term) way back starting in Chapter 18.
 For the intelligible species is the formal principle of the intellectual operation; even as the form of every agent is the principle of that agent’s proper operation. Now the intellectual operation of God is His essence, as we have shown. Wherefore something else would be the principle and cause of the divine essence, if the divine intellect understood by some intelligible species other than His essence: and this is in contradiction with what has been shown above.
Notes And this is the main point. A car operates by being in the form of a car, yes? And God operates by His form, His essence. A weak analogy would be to say that if God had a brain by which His intelligible species operated, He would have parts, which we have already proved He cannot. And we’d also have to explain how this “Brain of God” moved God’s intelligence around separate from His will. Sort of like how materialists have to prove the “Brain of Man” moves your intellect about separate from your will. The next two arguments illuminate this.
 Again. The intellect is made actually intelligent by the intelligible species: just as sense is made actually sentient by the sensible species. Hence the intelligible species is compared to the intellect as act to potentiality. And consequently if the divine intellect were to understand by a species other than itself, it would be in potentiality with respect to something: and this is impossible, as we have proved above.
 Moreover. An intelligible species that is accessory to the essence of the intellect in which it is, has an accidental being: for which reason our knowledge is reckoned among the accidents. Now in God there can be no accident, as proved above. Therefore there is no species in His intellect besides the divine essence…
Notes In  there is the proof, if you like, that animals are sentient, which means having the capability of sensing. This is only worth pointing out because of the modern understanding that all sentient creatures, including humans, are alike. This is so, but humans are not just sentient, but are also rational, possessing an intellect, and animals do not.
The second point is the ever-necessary distinction between act and potential. Only something in act, or that has actuality, can move a potential to act, i.e. can cause something to happen. In God there is no potentiality, in us there is. The second translation of  (linked above) puts it in better modern English, “an intelligible species in the intellect that is other than the intellect’s essence has an accidental being, which is why our knowledge is numbered among the accidents.” Some (I don’t think we can say all) of the knowledge we have is not essential to our being, but all of God’s knowledge is His essence.
 Moreover. God’s act of intelligence is His essence, as we have proved. Therefore if He understood by a species that is not His essence, it would be by something other than His essence. But this is impossible. Therefore He does not understand by a species that is not His essence.
Notes And that, I think, is enough for today. Review the terms act and potential, or actuality and potentiality, and form and accident. (All the posts in this category are found in the link at the top.) Next week it gets harder.
 Ch. xlv.
 Ch. xiii.
 Ch. xvi.
 Ch. xxiii.
 Ch. xlv.
 Ch. xxii.