Thomas Aquinas

“ Therefore just as in external judgment no man can pass sentence of excommunication on himself, so neither can he empower another to excommunicate him. On the other hand, in the tribunal of conscience he can give another the power to absolve him, though he cannot use that power himself.
It may also be replied that absolution in the tribunal of the confessional belongs principally to the power of the keys and consequently to the power of jurisdiction, whereas excommunication regards jurisdiction exclusively. And, as to the power of orders, all are equal, but not as to jurisdiction. ”

“ I answer that, That which has to do with singular matters is not equally in the power of all. Thus, even as besides the general principles of medicine, it is necessary to have physicians, who adapt those general principles to individual patients or diseases, according to their various requirements, so in every kingdom, besides that one who proclaims the universal precepts of law, there is need for others to adapt those precepts to individual cases, according as each case demands. ”

“ Hence a priest can absolve from a crime, but for the remission of the punishment he must refer the penitent to the superior, except in the case of excommunication, absolution from which should precede absolution from sin, for as long as a man is excommunicated, he cannot receive any sacrament of the Church.
Reply to Objection 3: This objection considers those cases in which superiors reserve the power of jurisdiction to themselves.
Art. 3 - Whether a man can use the keys with regard to his superior? Objection 1: It would seem that a man cannot use the keys in respect of a superior. ”