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 indulgences cannot be granted by one who is in mortal sin. For a stream can no longer flow if cut off from its source. Now the source of grace which is the Holy Ghost is cut off from one who is in mortal sin. Therefore such a one can convey nothing to others by granting indulgences.
Obj. 2: Further, It is a greater thing to grant an indulgence than to receive one. But one who is in mortal sin cannot receive an indulgence, as we shall show presently (Q. XXVI L, A. i). Neither, therefore, can he grant one.
On the contrary, Indulgences are granted in virtue of the power conferred on the prelates of the Church. Now mortal sin takes away, not power but goodness. Therefore one who is in mortal sin can grant indulgences.
I answer that, The granting of indulgences belongs to jurisdiction. But a man does not, through sin, lose jurisdiction. Consequently indulgences are equally valid, whether they be granted by one who is in mortal sin, or by a most holy person; since he remits punishment, not by virtue of his own merits, but by virtue of the merits laid up in the Church's treasury.
Reply Obj. 1: The prelate who, while in a state of mortal sin, grants an indulgence, does not pour forth anything of his own, and so it is not necessary that he should receive an inflow from the source, in order that he may grant a valid indulgence.
Reply Obj. 2: Further, To grant an indulgence is more than to receive one, if we consider the power, but it is less, if we consider the personal profit.
Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.