Summa Theologica

Having considered the qualities that belong to God in his essence, Aquinas begins a discussion of God considered as the Holy Trinity. He explains what it means for the second and third persons of the trinity (Son, and Holy Spirit) to “process” from the Father. He clarifies that procession (or generation) of the divine persons is not the same as biological procreation. It is a more interior and spiritual sort of thing, similar to the progression that occurs within human beings from understanding to mental conception to speech. 

Christian doctrine ascribes certain relationships among the persons of the Trinity. Aquinas asserts that these relations are real, not merely logical or metaphorical constructs. Further, in God, relation is not merely accidental but substantive. In other words, relation is identical to God’s nature and essence. Even so, the relations between Father, Son, and Holy Spirit are distinct. There are in fact four relations in the Trinity: paternity, filiation, spiration, and procession.

Aquinas begins his discussion of the persons of the Trinity by discussing personhood in general. What is a person? Aquinas starts with the definition offered by Boethius: “an individual