Thomas Aquinas

“ I answer that, By receiving Orders a man pledges himself to the Divine offices. And since no man can give what is not his, a slave who has not the disposal of himself, cannot be raised to Orders. If, however, he be raised, he receives the Order, because freedom is not required for the validity of the sacrament, although it is requisite for its lawfulness, since it hinders not the power, but the act only. The same reason applies to all who are under an obligation to others, such as those who are in debt and like persons. ”

“ I answer that, Those who are ordained are placed in a position of dignity over others. Hence by a kind of propriety it is requisite that they should be without reproach, not for the validity but for the lawfulness of the sacrament, namely that they should be of good repute, bedecked with a virtuous life, and not publicly penitent. And since a man's good name is bedimmed by a sinful origin, therefore those also who are born of an unlawful union are debarred from receiving orders, unless they receive a dispensation ”

“ But for the episcopate whereby a man receives power also over the mystical body, the act of accepting the pastoral care of souls is required; wherefore the use of reason is necessary for the validity of episcopal consecration. Some, however, maintain that the use of reason is necessary for the validity of the sacrament in all the Orders. ”