That’s Augustine’s term for the following argument:
Granted that there’s no absolute opposition between the Father/Son pairing, that still doesn’t resolve the tension between the Father’s un-begottenness and the Son’s begottenness; one could be un-begotten and not be a Father, which shows that we must consider these pairs in isolation from one another, and it’s the second, not the first, that damns “consubstantiality,” since that pair implies “absolute difference.”
He summarizes it this way: “Les mots père et fils supposent une relation entre les personnes divines, tandis que ceux de non-engendré et d’engendré tombent sur la personnalité elle-même, et s’entendent de la substance. Or être engendré et n’être pas engendré sont deux choses absolument différentes; donc en Dieu il y a diversité de substance” (De Trin. V.6.7).
Augustine’s solution is elegant: granting that “unbegotten” is not intrinsically related to “Father,” it nonetheless remains true that “begotten” is intrinsically related to “Son”: that the Son is begotten is solely a function of his being the Son of the Father, which is a term of a relation. The Son’s “begottenness” does not introduce diversity of substance into the Godhead, because it is not predicated of the Son according to substance.