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 seems that a man ought to have contrition for another's sin. For one should not ask forgiveness for a sin unless one is contrite for it. Now forgiveness is asked for another's sin in Ps. xviii. 13: From those of others spare thy servant. Therefore a man ought to be contrite for another's sins.
Obj. 2: Further, man is bound, out of charity, to love his neighbour as himself. Now, through love of himself, he both grieves for his ills, and desires good things. Therefore, since we are bound to desire the goods of grace for our neighbour, as for ourselves, it seems that we ought to grieve for his sins, even as for our own. But contrition is nothing else than sorrow for sins. Therefore man should be contrite for the sins of others.
On the contrary, Contrition is an act of the virtue of penance. But no one repents save for what he has done himself. Therefore no one is contrite for others' sins.
I answer that, The same thing is crushed (conteritur) which hitherto was hard and whole. Hence contrition for sin must needs be in the same subject in which the hardness of sin was hitherto: so that there is no contrition for the sins of others.
Reply Obj. 1: The prophet prays to be spared from the sins of others, in so far as, through fellowship with sinners, a man contracts a stain by consenting to their sins: thus it is written (Ps. xvii. 27): With the perverse thou wilt he perverted.
Reply Obj. 2: We ought to grieve for the sms of others, but not to have contrition for them, because not all sorrow for past sins is contrition, as is evident from what has been said already.
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Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.