The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 29

That the Holy Spirit is distinguished from the Son in this that he is from him.

It is also shown from the texts of the aforesaid Fathers that the Holy Spirit is distinguished from the Son, because he is from him. Gregory of Nyssa teaching dogmatically about the godhead says: “We confess the divine nature to be immutable; but we do not deny the difference between cause and thing caused”, that is, as explained above,  the distinction between the principle and that which is from the principle. Afterwards he adds: “We also recognize another difference, namely, between one who is the Relative from the First”, that is, the Son from the Father, and “the other who is from the Relative and from the First”, that is, the Spirit who is from the Father and the Son. 

It is clear, therefore, that in virtue of the first difference the Holy Spirit and Son are distinguished from the Father; in virtue of the second the Holy Spirit is distinguished from the Son, namely, because the Son proceeds from the Father not through the Spirit, but the Spirit proceeds from the Son. Similarly, Richard of St. Victor in book five on the Trinity  also shows the difference between the two processions from this that the Son proceeds from only one person, whereas the Holy Spirit proceeds from two. According to the views of both the only possible conclusion is that the Son and Holy Spirit would not be distinguished from one another unless one were from the other.