Augustine’s The Trinity unfolds in such large sweeps that it is easy to lose the flow of argument. As Books 3-4 weaves through discussions of theophanies and numerological musings on the proportion of 1 and 2, and the perfections of the number 6, it’s easy to forget what the question is. At the end, he arrives back at the point, the puzzle that he identified earlier. In Book 1, he said that people “want to understand how that utterance [at Jesus’ baptism] which was only the Father’s was caused by the three; and how that flesh in which only the Son was born of the virgin was created by the same three; and how that form of the dove in which only the Spirit appeared was fashioned by the trinity itself” (1.2.5). At the end of Book 4, he says that “the trinity together produced both the Father’s voice and the Son’s flesh and the Holy Spirit’s dove, though each of these things has reference to a single person” (4.5.30).
That suggests that the discussion of processions and missions is subordinate to the question of the inseparable operations of the Trinity. Even the question of the subordination of the sent to the sender seems to be part of the larger argument about inseparable operations. And that makes his solution to the subordinate puzzles - what makes the missions different from the theophanies? How could the Son and Spirit be sent to places where they already were? - all the more stunning.
The intention of the sendings is not that the Son and Spirit enter a place they had not been, since they were both already in the world into which they were sent, or because they were subordinate to the Father. The sendings are instead a revelation of origin. They are sent so that it will be revealed that they are sent from the Father: The Son was “sent . . . in virtue of the Son being from the Father, not the Father being from the Son” (4.5.27). More fully, “just as being born means for the Son his being from the Father, so his being sent means his being known to be from him. And just as for the Holy Spirit his being the gift of God means his proceeding from the Father, so hie being sent means his being known to proceed from him” (4.5.29).
The sendings thus reveal a twofold reality about God: First, that the Father is the origin of the Son and Spirit, and, second, that the Son and Spirit are differentiated as sent from the Father who is sender. The genius of this point is that this is how Augustine proves the inseparable operations of the triune persons. The examples of the Father’s voice, and the Son’s flesh, and the Spirit’s dove - the examples of the sendings of Son and Spirit - show “how the three are inseparably at work in each of the things which are mentioned as having the proper function of manifesting the Father or the Son or the Holy Spirit” (4.5.30). The inseparable operations are demonstrated precisely in the difference-revealing sendings, because the difference-revealing sendings locate the unity of God’s working in the fact that all God’s works originate from the Father.
It is remarkable too that this is the very point where Augustine first (I believe) introduces the “psychological analogies”: “when I name my memory, understanding, and will, each name refers to a single thing, and yet each of these single names is the product of all three; there is no one of these three names which my memory and understanding and will have not produced together. So too the trinity together produced both the Father’s voice an the Son’s flesh and the Spirit’s dove” (4.5.30). The psychological analogies are designed to illustrate the inseparable operations of the triune persons, but they illustrate this precisely insofar as it is first established that “each name refers to a single thing.” The inseparable operations of the mind’s faculties are revealed in their differentiation.