The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 30

That the distinction of persons should be according to some order of nature.

Similarly, the distinction of persons should rest on an order which is natural, as Augustine says.  Hence, Athanasius in his letter to Serapion likens the order of distinct persons to a chain: “Indeed, just as he who pulls the head link of a chain pulls also its middle link, and against the Father, the opposite extremity, so he who blasphemes also against the Spirit, the third person, blasphemes also against the Son, the middle link, and against the Father, the opposite link, the head of the chain of the triune, distinct, unconfused divine order. Contrariwise, he who believes and receives the Spirit as God, receives God and the Son whose he is and from whom he is, just as one who holds one end of a chain pulling it toward himself, holds the middle and through the middle grasps the other end.” 

For this reason he also says in the same letter: “The Spirit Paraclete, the term of the blessed and transcendent divine order, infallibly constitutes the proper termination of this order in himself by his own hypostasis, just as the Father himself without principle contains the head and frontal origin of this order. The Son, however, occupies the intermediate position of this order between its extremes, namely the Father and the Holy Spirit.” And shortly after: “The Father from himself, as origin of the triune divine order, through the medium of his begotten Son established by a natural property the term of this very order in the third person, the spirated Spirit.” 

Cyril also says in the Thesaurus: “The Holy Spirit is by nature from the Son and is sent by him to the creature, to work the renewal of the Church and to be the term of the Holy Trinity.” And he concludes: “If this is so, then God from God the Son is the Holy Spirit.”  For if the Holy Spirit were not from the Son, the Holy Spirit would no more be the term of the Trinity than the Son, nor would the order of the Trinity be likened to a chain but rather to a triangle.

Richard of St. Victor also touches on this argument in book five on the Trinity,  where he shows that among the divine persons there can be only one person from another person from whom another person does not proceed, nor can there be two persons from only one person. Either of these alternatives would be in contradiction to the aforesaid order among the divine persons, but both would be posited if the Holy Spirit did not proceed from the Son.

Cyril in his Thesaurus  explains this order among the divine persons via another analogy employed on the authority of Holy Scripture, which in the Gospel calls the Holy Spirit the finger of God: If I by the finger of God drive out demons (Luke 11:20), and the parallel passage in another Gospel: If I in the Spirit of God, etc. (Matt. 12: 28). The Son, however, is called the arm of God: Clothe yourself with strength, O arm of the Lord (Is. 51: 9). Cyril says: “As the arm and hand exist naturally from the body and prolong it, and as the finger extends naturally from the hand, so from God the Father, as his arm and hand, the Son naturally arises by generation God from God, and from the Son as from the natural hand of the Father God the Holy Spirit called finger is produced, flowing forth naturally.” 

To conclude, therefore, the Holy Spirit proceeds from the Son for reasons affirmed equally by the Latin and Greek Doctors.