“Si donc l’Esprit-Saint qui est donné, a pour principe celui qui le donne, parce qu’il ne procède que de lui, il faut avouer qu’à l’égard de ce divin Esprit le Père et le Fils sont un seul et unique principe, et non deux principes.”
—De Trin. V.14.15
If to give the Holy Spirit is to be the principle of his procession, then the Holy Spirit must proceed from Father and Son. So far, so good, but Augustine (in this section at least) doesn’t make the crucial distinction that saves Thomas’s account from modalism: the Holy Spirit certainly proceeds from the Father and the Son, but not equally from both, for that would mean his procession from the only thing that Father and Son share, which is the divine essence, thus forcing us to posit a monadic, “real” God behind the persons. Thomas cuts this Gordian knot by noting arguing that the Father, as sole monarch, gifts the Spirit just as he begets the Son, but that, in the case of the Spirit, the Father shares this gifting with the Son.