Objection 1. It would seem that one may confess through another, or by writing. For confession is necessary in order that the penitent's conscience may be made known to the priest. But a man can make his conscience known to the priest, through another or by writing. Therefore it is enough to confess through another or by writing.
Objection 2. Further, some are not understood by their own priests on account of a difference of language, and consequently cannot confess save through others. Therefore it is not essential to the sacrament that one should confess by oneself, so that if anyone confesses through another in any way whatever, it suffices for his salvation.
Objection 3. Further, it is essential to the sacrament that a man should confess to his own priest, as appears from what has been said (8, 5). Now sometimes a man's own priest is absent, so that the penitent cannot speak to him with his own voice. But he could make his conscience known to him by writing. Therefore it seems that he ought to manifest his conscience to him by writing to him.
On the contrary, Man is bound to confess his sins even as he is bound to confess his faith. But confession of faith should be made "with the mouth," as appears from Romans 10:10: therefore confession of sins should also.
Further, who sinned by himself should, by himself, do penance. But confession is part of penance. Therefore the penitent should confess his own sins.
I answer that, Confession is not only an act of virtue, but also part of a sacrament. Now, though, in so far as it is an act of virtue it matters not how it is done, even if it be easier to do it in one way than in another, yet, in so far as it is part of a sacrament, it has a determinate act, just as the other sacraments have a determinate matter. And as in Baptism, in order to signify the inward washing, we employ that element which is chiefly used in washing, so in the sacramental act which is intended for manifestation we generally make use of that act which is most commonly employed for the purpose of manifestation, viz. our own words; for other ways have been introduced as supplementary to this.
Reply to Objection 1. Just as in Baptism it is not enough to wash with anything, but it is necessary to wash with a determinate element, so neither does it suffice, in Penance, to manifest one's sins anyhow, but they must be declared by a determinate act.
Reply to Objection 2. It is enough for one who is ignorant of a language, to confess by writing, or by signs, or by an interpreter, because a man is not bound to do more than he can: although a man is not able or obliged to receive Baptism, except with water, which is from an entirely external source and is applied to us by another: whereas the act of confession is from within and is performed by ourselves, so that when we cannot confess in one way, we must confess as we can.
Reply to Objection 3. In the absence of one's own priest, confession may be made even to a layman, so that there is no necessity to confess in writing, because the act of confession is more essential than the person to whom confession is made.
Source: Summa Theologica (New Advent)