Western Trinitarian theology develops from Augustine, but because Augustine is complex the Western tradition develops along different - equally Augustinian - pathways. That is the argument of Dennis Ngien’s 2005 study of the filioque in medieval theology. Anselm, he says, develops the “essentialist” aspect of Augustine’s doctrine, starting with the united essence and working out distinctions among the persons. Unlike Augustine, Anselm concludes that “the origin of the Holy Spirit is not rooted in the mutual love between the Father and the Son as in Augustine, but in love as the eternal essence shared by both - viz., their one Godness of love.”

Richard of St. Victor, by contrast, develops the “personalist” strain of Augustine. Love, Augustine had noted, is triadic, but instead of seeing the Spirit as the love binding the Father and Son, Richard sees the Spirit as the One co-loved by the Father and Son. As Ngien puts it, “Since God is supremely loving, and only God is deserving of supreme love, the infinite love which is God presupposes and infinite object, even when there are no creatures. Love consists of three levels, moving from self-love (Father) to charity, in which a second is loved (Son), to complete charity, in which a third is mutually loved by the pair (Spirit). ‘The perfection of the divine love is revealed by the fact that it is neither self-love nor merely the reciprocal love of two for each other, but a love intrinsically oriented to community.’” (The last is a quotation from Colin Gunton.)

In Ngien’s view, Aquinas combines the two straings: “On the one hand, Aquinas is with Anselm in giving priority to the solitary love as the formal reason of spiration; on the other hand, he does not distance himself from Augustine in affirming the Spirit as the fruit of the mutual love of Father and the Son for each other, and by way of John Damascene, he accentuates the personalist approach to the divine mystery.” Holding the two in balance, he affirms “not only the Spirit as proceeding from the love of the Infinite Being for itself, but also as the mutual love of the Father and the Son for each other.” Ngien suggests that Aquinas might offer a way of affirming the filioque that will satisfy Eastern theologians.