From the Supplement, compiled after Aquinas’s death from his Commentary on the Sentences and transcribed here from the 1917 to 1922 printings. About the Supplement.
Text: Summa Theologiae, Supplement to the Third Part (Supplementum Tertiae Partis), transcribed from Internet Archive scans, translated by the Fathers of the English Dominican Province. The Supplement, compiled after Aquinas’s death from his earlier Commentary on the Sentences, in the same English Dominican translation as the Gutenberg Parts. Transcribed by Bibliotheca Catholica from the OCR of four Internet Archive scans of the pre-1931 printings (Questions 1 to 33 from the 1917 Washbourne and Benziger volume, 34 to 68 from the 1922 volume, 69 to 86 from the 1921 volume, and 87 to 99 from the 1922 Benziger volume), with OCR errors corrected and doubtful passages marked, not rewritten. 99 questions and 446 articles. The two appendices printed after Question 99 are not included. Sources, SHA-256 of each OCR file, and the method are in the source record. Rights: Public domain (US)
The Supplement is not a Project Gutenberg text. It is Bibliotheca Catholica’s transcription of public-domain scans at the Internet Archive; obvious OCR errors were corrected, and passages marked “OCR uncertain” could not be confidently corrected, so check them against the linked scan page.
Objection 1: It would seem that spiritual relationship does not pass to the godfather's carnal children. For no degrees are assigned to spiritual relationship. Yet there would be degrees if it passed from father to son, since the person begotten involves a change of degree, as stated above (Q. LV., A. 5). Therefore it does not pass to the godfather's carnal sons.
Obj. 2: Further, Father and son are related in the same degree as brother and brother. If therefore spiritual relationship passes from father to son, it will equally pass from brother to brother: and this is false.
On the contrary, This is proved by authority quoted in the text (iv. Sent. D. 42.)-
I answer that, A son is something of his father and not conversely (Ethic, viii. 12): wherefore spiritual relationship passes from father to his carnal son and not conversely. Thus it is clear that there are three spiritual relationships: one called spiritual fatherhood between godfather and godchild; another called co-paternity between the godparent and carnal parent of the same person; and the third is called spiritual brotherhood, between godchild and the carnal children of the same parent. Each of these hinders the contracting of marriage and voids the contract.
Reply Obj. 1: The addition of a person by carnal generation entails a degree with regard to a person connected by the same kind of relationship, but not with regard to one connected by another kind of relationship. Thus a son is connected with his father's wife in the same degree as his father, but by another kind of relationship. Now spiritual relationship differs in kind from carnal. Wherefore a godson is not related to his godfather's carnal son in the same degree as the latter's father is related to him, through whom the spiritual relationship is contracted. Consequently it does not follow that spiritual relationship admits of degrees.
Reply Obj. 2: A man is not part of his brother as a son is of his father. But a wife is part of her husband, since she is made one with him in body. Consequently the relationship does not pass from brother to brother, whether the brother be born before or after spiritual brotherhood.
Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.