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 suffrages avail the children who are in limbo. For they are not detained there except for another's sin. Therefore it is most becoming that they should be assisted by the suffrages of others.
Obj. 2: Further, In the text (iv. Sent. D. 45) the words of Augustine (Enchir. ex.) are quoted: The suffrages of the Church obtain forgiveness for those who are not very bad. Now children are not reckoned among those who are very bad, since their punishment is very light. Therefore the suffrages of the Church avail them.
On the contrary, The text (ibid) quotes Augustine as saying (Serm. xxxii. De Verb Ap.) that suffrages avail not those who have departed hence without the faith that works by love. Now the children departed thus. Therefore suffrages avail them not.
I answer that, Unbaptized children are not detained in limbo save because they lack the state of grace. Hence, since the state of the dead cannot be changed by the works of the living, especially as regards the merit of the essential reward or punishment, the suffrages of the living cannot profit the children in limbo.
Reply Obj. 1: Although original sin is such that one person can be assisted by another on its account, nevertheless the souls of the children in limbo are in such a state that they cannot be assisted, because after this life there is no time for obtaining grace.
Reply Obj. 2: Augustine is speaking of those who are not very bad, but have been baptized. This is clear from what precedes: Since these sacrifices, whether of the altar or of any alms whatsoever, are offered for those who have been baptized etc.
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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.