From the Second Book.

8. The individual names uttered by me can neither be separated from one another, nor parted. [729] I spoke of the Father, and before I made mention of the Son I already signified Him in the Father. I added the Son; and the Father, even although I had not previously named Him, had already been absolutely comprehended in the Son. I added the Holy Spirit; but, at the same time, I conveyed under the name whence and by whom He proceeded. But they are ignorant that neither the Father, in that He is Father, can be separated from the Son, for that name is the evident ground of coherence and conjunction; nor can the Son be separated from the Father, for this word Father indicates association between them. And there is, moreover, evident a Spirit who can neither be disjoined from Him who sends, nor from Him who brings Him. How, then, should I who use such names think that these are absolutely divided and separated the one from the other?

After a few words he adds: --

9. Thus, indeed, we expand the indivisible Unity into a Trinity; and again we contract the Trinity, which cannot be diminished, into a Unity.