“And we have from the true faith that God [the Son] is from God by generation, and that God [the Spirit] is from God by procession. But since there is nothing of God outside of God, when God is generated from God, or when God proceeds from God, the generation and procession do not go outside of God but remain in him.
Therefore, since God in God is only one God, when God is generated from God, there is only one God who generates and is generated. And when God proceeds from God, there is only one God who proceeds and from whom he proceeds. And so it inevitably follows that, since God has no parts, and the whole of him is whatever he is, the whole God, one and the same and not one and another, is the Father, is the Son, is the Holy Spirit.
And so, by analogy to one human being, the Father and the Son and the Holy Spirit preserve the uniqueness in the divine nature by reason of the fact that there is only one God if God from God is God in God. And like different human persons, the Father and the Son and the Holy Spirit have plurality of persons by reason of names signifying the relations of generation and procession, since one who is from another, and one from whom another is cannot be one and the same if there is God from God, whether by generation or procession.
Nevertheless,we should note that neither is there God without the persons, nor the persons without God, and that we sometimes attribute to individual persons the characteristics proper to the persons, sometimes characteristics common to the others as if proper to one. For example, when we say that, of the three persons, only the Father is one who is from no one, only the Son is one who is from one, and from whom one is, only the Holy Spirit is one from whom no one is, we attribute to individual persons proper characteristics denominating the individual persons.
And when we read, ‘No one knows the Son except the Father,and no one knows the Father except the Son’ [Matt.II:27], and, ‘No one except the Spirit of God knows the things belonging to God’ [I Cor.2:II], although Scripture seems to deny about the other persons what it says about one person, the characteristics attributed to individual persons as if proper to them are none the less common to all the persons. For example, the Father and the Son know their very selves, and the Holy Spirit knows the Father and the Son.“
-Anselm of Canterbury, On the Procession of the Holy Spirit, 16.