Likewise we believe also in one Holy Spirit, the Lord and Giver of Life: Who proceeds from the Father and rests in the Son: the object of equal adoration and glorification with the Father and Son, since He is co-essential and co-eternal: the Spirit of God, direct, authoritative, the fountain of wisdom, and life, and holiness: God existing and addressed along with Father and Son: uncreate, full, creative, all-ruling, all-effecting, all-powerful, of infinite power, Lord of all creation and not under any lord: deifying, not deified: filling, not filled: shared in, not sharing in: sanctifying, not sanctified: the intercessor, receiving the supplications of all: in all things like to the Father and Son: proceeding from the Father and communicated through the Son, and participated in by all creation, through Himself creating, and investing with essence and sanctifying, and maintaining the universe: having subsistence, existing in its own proper and peculiar subsistence, inseparable and indivisible from Father and Son, and possessing all the qualities that the Father and Son possess, save that of not being begotten or born. For the Father is without cause and unborn: for He is derived from nothing, but derives from Himself His being, nor does He derive a single quality from another. Rather He is Himself the beginning and cause of the existence of all things in a definite and natural manner. But the Son is derived from the Father after the manner of generation, and the Holy Spirit likewise is derived from the Father, yet not after the manner of generation, but after that of procession. And we have learned that there is a difference between generation and procession, but the nature of that difference we in no wise understand. Further, the generation of the Son from the Father and the procession of the Holy Spirit are simultaneous.

All then that the Son and the Spirit have is from the Father, even their very being: and unless the Father is, neither the Son nor the Spirit is. And unless the Father possesses a certain attribute, neither the Son nor the Spirit possesses it: and through the Father, that is, because of the Father’s existence, the Son and the Spirit exist, and through the Father, that is, because of the Father having the qualities, the Son and the Spirit have all their qualities, those of being unbegotten, and of birth and of procession being excepted. For in these hypostatic or personal properties alone do the three holy subsistences differ from each other, being indivisibly divided not by essence but by the distinguishing mark of their proper and peculiar subsistence.

Further we say that each of the three has a perfect subsistence, that we may understand not one compound perfect nature made up of three imperfect elements, but one simple essence, surpassing and preceding perfection, existing in three perfect subsistences. — Book I, Chapter 8, Concerning the Holy Trinity.