In Epistle 11, Augustine attempts to explain an apparent contradiction in the Catholic faith. On the one hand, all of God does all that God does, since the Persons of the Trinity are inseparable and act inseparably: “For the union of Persons in the Trinity is in the Catholic faith set forth and believed, and by a few holy and blessed ones understood, to be so inseparable, that whatever is done by the Trinity must be regarded as being done by the Father, and by the Son, and by the Holy Spirit together; and that nothing is done by the Father which is, not also done by the Son and by the Holy Spirit; and nothing done by the Holy Spirit which is not also done by the Father and by the Son; and nothing done by the Son which is not also done by the Father and by the Holy Spirit.”
On the other hand, only the Son was incarnate. How is that possible?
Augustine’s answer begins with a discussion of the nature of “nature” or “substance.”
Every substance, he says, must be , must be something , and must continue being what it is, “as far as possible.” He labels these “origin,” “form,” and “permanence”: The first of these three presents the original cause of nature from which all things exist; the second presents the form according to which all things are fashioned and formed in a particular way ( speciem per quam fabricantur, et quodammodo formantur omnia ); the third presents a certain permanence, so to speak, in which all things are.”
It is not immediately obvious how this is addressing the question. But Augustine sees quite a direct analogy between the being, the being-something, and the being-something-over-time of things and the tri-Personal nature of God. If it were possible for things to be without being something, or to be something without being, he argues, “then it is also possible that in that Trinity one Person can do something in which the others have no part.” The implication is that the three features of created substances correspond to the three Persons, a hint that becomes clear in the next section, where Augustine speaks of the “mode of existence (Species— the second of the three above named) which is properly ascribed to the Son” ( species quae proprie Filio tribuitur ).
The answer to the specific question arises from Augustine’s explanation that the Son’s particular contribution to created existence (species) “has to do with training, and with a certain art ( ea pertinet etiam ad disciplinam, et ad artem quamdam ), if I may use that word in regard to such things, and with the exercise of intellect, by which the mind itself is molded in its thoughts upon things.” The form of things impresses itself on our minds, so that as we contemplate them our souls are formed by them ( animus rerum cogitatione formatur ). The Son, the source of the form of things, becomes man so that He can impress form onto human nature: “since by that assumption of human nature the work accomplished was the effective presentation to us of a certain training in the right way of living, and exemplification of that which is commanded, under the majesty and perspicuousness of certain sentences, it is not without reason that all this is ascribed to the Son.”
The specific answer here is of interest, but the way that Augustine arrives here is also of interest. The indivisibility of the opera ad extra does not blur differences between the Persons; rather, each Person makes a peculiar contribution to the one act of creating a created nature.