be brought under their cognisance, in order that, using the power of the keys, they might pronounce sentence of remission or retention"; and further, that priests " could not exercise this power of judgment without examination of the case "; and again, in the ninth canon of the same Session, the Council anathematises "any one who shall say that the sacramental absolution of the priest is not a judicial act." Every time that a priest is seated in the confessional, he is there as a judge. He must, then, possess jurisdiction as well as order; otherwise his acts are invalid, and his absolution goes for nothing. He must be in fact either the ecclesiastical superior of his penitent, or the delegate of that superior. Being a judge, he is bound to decide according to the law of the court where he sits — the court of conscience it is called. The law there current presents many nice points for decision. The study of these, as I have said, is casuistry. It is essential to the training of a priest. It is matter of professional interest to him, and occasionally of keen discussion, as the treatment of wounds is to a surgeon.
Link (here) to the essay entitled Casuistry by Fr. Joseph Rickaby, S.J.