When discussing Aquinas' proofs for God's existence, Edward Feser always makes the point that when Aquinas asserts that a series of cause and effects cannot be infinite, Aquinas is referring to a causal series per se--or an essentially ordered causal series--as opposed to a causal series per accidens.

When looking into Aquinas' writings, however, I have not found this distinction made at all. The closest thing I have found is that Aquinas holds that the eternity of the world cannot be disproven from reason alone, but it requires divine revelation, and if he holds this view then he certainly cannot be discussing temporally ordered series of cause and effect in his Second Way.

I also know that the distinction between the two kinds of cause and effect was defined by Duns Scotus. My question is whether Aquinas' idea of cause and effect was a causal series per se or is that something that later Scholastics and Thomists tacked onto his arguments to make them more sound.