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?