A common misconception of St. Augustine’s doctrine of the filioque is that, due to neo-platonic influences, he arrives at the doctrine by approaching the doctrine of the Trinity at the starting point of the unity of the divine essence. Since the Son is of the same essence as the Father, and the Spirit proceeds from the Father, therefore the Spirit must likewise proceed from the Son. While this sort of theological musing isn’t entirely absent from St. Augustine’s thought it doesn’t stand to be the central argument for his proposition of the filioque. The doctrine of the filioque shows up periodically throughout his work The Trinity and what we can find is that St. Augustine arrives at the doctrine of the filioque through the implementation a number of hermenuetical principles.
The first principle is St. Augustine’s application of either the divine nature or the human nature of Christ as a hermenuetical lens. This is related to his doctrine of the filioque because in The Trinityi he applies this tactic to Christ’s declaration in Jn. 7:16 that, “My doctrine is not mine,” to refer to Christ’s human nature while at the same time St. Augustine affirms that Christ’s doctrine is His own according to His divine nature. Elsewhereii St. Augustine interprets Jn. 15:26 in the same manner, even using his interpretation of Jn. 7:16 as an aid,
“So if the Holy Spirit proceeds from both the Father and the Son, why did the Son say he proceeds from the Father (Jn. 15:26)? Why indeed, do you suppose, unless it was the way he was accustomed to refer even what was his very own to him from whom he had his very self? – for example, that other thing he said, My teaching is not mine but him who sent me (Jn. 7:16). If in this case we can accept that it is his teaching, which he says however is not his but the Father’s, how much more should we accept in our other case that the Holy Spirit also proceeds from him, seeing that he said, he proceeds from the Father, without also saying ‘he does not proceed from me’?”iii
Another way St. Augustine arrives at the doctrine of the filioque is through his doctrine of the mission of the Son and the Spirit. In Books 2-3 of The Trinity he analyzes the Old Testament theophanies and concludes that in every case God manifested Himself through a created means of mediation (in these cases the mediators were angels). In Book 4 he argues that in the case of the Incarnation God reveals Himself through the created human flesh of Christ and that at Pentecost the Spirit reveals Himself through the created tongues of fire. He then works backwards to see the economic missions of the Son and the Spirit to reflect their eternal begetting and procession. Since the Father is unoriginate He has no mission while the Son and the Spirit come from the Father therefore they are the ones who are sent economically. Through his doctrine of the mission of the Son and the Spirit St. Augustine notes how Christ says that He will send the Spirit and how He breathes the Spirit onto His disciples and likewise works backwards that since the Son sends the Spirit economically from the Father there the Spirit must also proceed from the Son with the Father.
More examples could be given from a careful reading of The Trinity to show that St. Augustine’s doctrine of the filioque has a greater bases upon his interpretation of scripture than it does in neo-platonic philosophy. While it may be possible to dispute St. Augustine’s conclusions, or at the very least to ask him what we should say regarding the eternal begetting of the Son if He was economically begotten by the Spirit according to the ways he has applied his hermenuetical lenses, we should at the very least discuss his doctrine of the filioque on his own terms rather than to dismiss it as merely being the result of a neo-platonic approach to the Trinity rather than to recognize the biblical reasons he provides for his doctrine.