8 To avoid prolixity, therefore, let me refer to the opinion which you, I believe, entertain, viz. that God even now makes each soul for each individual at the time of birth. To meet the objection to this view which might be taken from the fact that God finished the whole work of creation on the sixth day and rested on the seventh day, you quote the testimony of the words in the gospel, “My Father works hitherto, and I work.” This you have written in your letter to Marcellinus, in which letter, moreover, you have most kindly condescended to mention my name, saying that he had me here in Africa, who could more easily explain to him the opinion held by you. But had I been able to do this, he would not have applied for instruction to you, who were so remote from him, though perhaps he did not write from Africa to you. For I know not when he wrote it; I only know that he knew well my hesitation to embrace any definite view on this subject, for which reason he preferred to write to you without consulting me. Yet, even if he had consulted me, I would rather have encouraged him to write to you, and would have expressed my gratitude for the benefit which might have been conferred on us all, had you not preferred to send a brief note, instead of a full reply, doing this, I suppose, to save yourself from unnecessary expenditure of effort in a place where I, whom you supposed to be thoroughly acquainted with the subject of his inquiries, was at hand. Behold, I am willing that the opinion which you hold should be also mine; but I assure you that as yet I have not embraced it.
9. You have sent to me scholars, to whom you wish me to impart what I have not yet learned myself. Teach me, therefore, what I am to teach them; for many urge me vehemently to be a teacher on this subject, and to them I confess that of this, as well as of many other things, I am ignorant, and perhaps, though they maintain a respectful demeanour in my presence, they say among themselves: “Are you a master in Israel, and know not these things?” a rebuke which the Lord gave to one who belonged to the class of men who delighted in being called Rabbi; which was also the reason of his coming by night to the true Teacher, because perchance he, who had been accustomed to teach, blushed to take the learner's place. But, for my own part, it gives me much more pleasure to hear instruction from another, than to be myself listened to as a teacher. For I remember what He said to those whom, above all men, He had chosen: “But be not ye called Rabbi, for one is your master, even Christ.” Nor was it any other teacher who taught Moses by Jethro, Cornelius by Peter the earlier apostle, and Peter himself by Paul the later apostle; for by whomsoever truth is spoken, it is spoken by the gift of Him who is the Truth. What if the reason of our still being ignorant of these things, and of our having failed to discover them, even after praying, reading, thinking, and reasoning, be this: that full proof may be made not only of the love with which we give instruction to the ignorant, but also of the humility with which we receive instruction from the learned?
10. Teach me, therefore, I beseech you, what I may teach to others; teach me what I ought to hold as my own opinion; and tell me this: if souls are from day to day made for each individual separately at birth, where, in the case of infant children, is sin committed by these souls, so that they require the remission of sin in the sacrament of Christ, because of sinning in Adam from whom the sinful flesh has been derived? Or if they do not sin, how is it compatible with the justice of the Creator, that, because of their being united to mortal members derived from another, they are so brought under the bond of the sin of that other, that unless they be rescued by the Church, perdition overtakes them, although it is not in their own power to secure that they be rescued by the grace of baptism? Where, therefore, is the justice of the condemnation of so many thousands of souls, which in the deaths of infant children leave this world without the benefit of the Christian sacrament, if being newly created they have, not through any preceding sin of their own, but by the will of the Creator, become severally united to the individual bodies to animate which they were created and bestowed by Him, who certainly knew that every one of them was destined, not through any fault of its own, to leave the body without receiving the baptism of Christ? Seeing, therefore, that we may not say concerning God either that He compels them to become sinners, or that He punishes innocent souls and seeing that, on the other hand, it is not lawful for us to deny that nothing else than perdition is the doom of the souls, even of little children, which have departed from the body without the sacrament of Christ, tell me, I implore you, where anything can be found to support the opinion that souls are not all derived from that one soul of the first man, but are each created separately for each individual, as Adam's soul was made for him.
Source: Letters (New Advent)