Another analogy, which is less known although it is still keener, is the one pointed out by St. Augustine in his Confessions. It seems that for some reason it has attracted much less attention than other examples of triunity in various objects, cited in abundance by the same St. Augustine in his book de Trinitate, which belong to the same external and irrelevant analogies of which I spoke above. In the Confessions, St. Augustine states the following: In our spirit we must distinguish its simple immediate being (esse), its knowing (scire), and its willing (velle); these three acts are identical not only by their content, in so far as the extant one knows and wills himself, but their unity goes far deeper: each of them contains in itself the other two in their own characteristic quality, and, consequently, each internally contains already the whole fullness of the triune spirit. Indeed, in the first place, I am but not simply am — I am the one who knows and wills (sum sciens et volens); consequently, here my being as such already contains in itself both knowledge and will, secondly, if I know then I know, or am conscious of my being as well as of my will, I know or am conscious of the fact that I am and that I will (scio me esse et velle); thus here also, in knowledge, as such, or under the form (in the attribute) of knowledge, both being and will are contained; thirdly and finally I wish myself yet not simply myself, but myself as existing and knowing, I will my existence and knowledge (volo me esse a scire); consequently, the form of the will also contains in its attribute being and knowledge. In other words, each of these three fundamental acts of the spirit is completed in itself by the other two, and thus becomes individualized into a hill triune being.

This consideration approaches the truth of the triunity of God very closely and can serve as a natural transition to the further development of this truth, namely, in regard to the specific individual relations of the three divine subjects to the single essence or idea, which they actualize and in which they themselves become concretely realized.

St. Augustine develops his trinitarian system in various works, but primarily in his main work, De Trinitate. The latter represents the most systematic and, in this sense, the most remarkable treatise on the Holy Trinity and, in particular, on the Holy Spirit that exists in the patristic literature. Its distinguishing features are homoousianism, which is its point of departure, and relativism in its conception of trinitarity. St. Augustine is thus the true father of the Western type of trinitarian theology. In other words, his point of departure is not the trinity of the hypostases, as it is for the Cappadocians, but the unity of the ousia (essentia) (De Trinitate 1.2). His thought also relies on Tertullian's impersonalistic tradition, but without the latter's subordinationism. The unity of the Holy Trinity in the three hypostases is guaranteed precisely by this unity of substance (1.4.7).

In this one substance the three hypostases are distinguished "non secundum substantiam, sed secundum relativum" [not according to substance but according to relation] (5.5.6). The hypostases exist as mutual relations. The initial axiom of Catholic triadology, the doctrine of the hypostases as relations, is expressed precisely here. The Father is defined in relation to the Son as the Engendering One in relation to the Engendered One, while the Father and the Son are defined in relation to the Holy Spirit as Spirator in relation to Spiratus. This serves as the basis of the Augustinian doctrine of the Filioque. Here too the main principle is the unity of substance: "It must be admitted that the Father and the Son are a Beginning of the Holy Spirit, not two Beginnings; but as the Father and Son are one God, and one Creator, and one Lord relatively to the creature, so are they one Beginning relatively to the Holy Spirit" (5.14.15).

Augustine's point of departure in the doctrine of the Holy Trinity gives him a greater grasp than the Eastern theologians have of the problem of the connection of the Holy Trinity with respect to the interrelations of the hypostases; the Eastern theologians replace this connection to a certain degree by the mere juxtaposition of the hypostases. On this pathway, Augustine makes a true discovery in the trinitarian and pneumatological theology: He is the first to express the idea, wholly foreign to Eastern theology, of the Holy Trinity as Love. Here he clarifies a special significance of the Third hypostasis, namely, that this hypostasis is love itself, the connection of love, amor or dilectio. He takes as his point of departure an abstract schema of love which distinguishes between amans, quod amatur, et amor [the loving one, the loved one, and love] (De Trinitate 8.10, 14; 9.11). The Holy Spirit is, in the Holy Trinity, the love of Both, of the Father and of the Son. "And if the love by which the Father loves the Son, and the Son loves the Father, ineffably demonstrates the communion of both, what is more suitable than that He should be specially called love, who is the Spirit common to both?" (15.19) [This translation is taken from A Select Library of Nicene and Post-Nicene Fathers, vol. 3, p.219. — Trans.]. (From this Augustine also draws a conclusion related to the Filioque.)