I answer that, Since a member is part of the whole human body, it is for the sake of the whole, as the imperfect for the perfect. Hence a member of the human body is to be disposed of according as it is expedient for the body. Now a member of the human body is of itself useful to the good of the whole body, yet, accidentally it may happen to be hurtful, as when a decayed member is a source of corruption to the whole body. Accordingly so long as a member is healthy and retains its natural disposition, it cannot be cut off without injury to the whole body. But as the whole of man is directed as to his end to the whole of the community of which he is a part, as stated above (Question [61], Article [1]; Question [64], Articles [2],5), it may happen that although the removal of a member may be detrimental to the whole body, it may nevertheless be directed to the good of the community, in so far as it is applied to a person as a punishment for the purpose of restraining sin. Hence just as by public authority a person is lawfully deprived of life altogether on account of certain more heinous sins, so is he deprived of a member on account of certain lesser sins. But this is not lawful for a private individual, even with the consent of the owner of the member, because this would involve an injury to the community, to whom the man and all his parts belong. If, however, the member be decayed and therefore a source of corruption to the whole body, then it is lawful with the consent of the owner of the member, to cut away the member for the welfare of the whole body, since each one is entrusted with the care of his own welfare. The same applies if it be done with the consent of the person whose business it is to care for the welfare of the person who has a decayed member: otherwise it is altogether unlawful to maim anyone.