What does Aquinas mean when he refers to things being "identical numerically" or "specifically identical" with things?

Question

In his discussion on the simplicity of God in Summa Theologica, Thomas Aquinas considers the question of "whether God enters into the composition of other things":

Objection 1: It seems that God enters into the composition of other things, for Dionysius says (Coel. Hier. iv): "The being of all things is that which is above being --- the Godhead." But the being of all things enters into the composition of everything. Therefore God enters into the composition of other things...

... I answer that... it is not possible for God to enter into the composition of anything, either as a formal or material principle. First, because God is the first efficient cause. Now the efficient cause is not identical numerically with the form of the thing caused, but only specifically: for man begets man. But primary matter can be neither numerically nor specifically identical with an efficient cause; for the former is merely potential, while the latter is actual.

What does he mean by this? In particular, what it mean for something to be "identical numerically" with something, and what does it mean to be "specifically identical" with something?

Answer 1

The efficient cause is not numerically identical with the effect because the "things" involved into the "production process" are different individials : the father of John generates John but he is a different individual :

the father is not numerically identical with the son.

But the father, in order to produce a man must be himself a man, i.e. he must belong to the same species of the "generated" :

the father is specifically identical with the son.