Objection 1. It would seem that in no case can it be lawful to maim anyone. For Damascene says (De Fide Orth. iv, 20) that "sin consists in departing from what is according to nature, towards that which is contrary to nature." Now according to nature it is appointed by God that a man's body should be entire in its members, and it is contrary to nature that it should be deprived of a member. Therefore it seems that it is always a sin to maim a person.
Objection 2. Further, as the whole soul is to the whole body, so are the parts of the soul to the parts of the body (De Anima ii, 1). But it is unlawful to deprive a man of his soul by killing him, except by public authority. Therefore neither is it lawful to maim anyone, except perhaps by public authority.
Objection 3. Further, the welfare of the soul is to be preferred to the welfare of the body. Now it is not lawful for a man to maim himself for the sake of the soul's welfare: since the council of Nicea [P. I, sect. 4, can. i] punished those who castrated themselves that they might preserve chastity. Therefore it is not lawful for any other reason to maim a person.
On the contrary, It is written (Exodus 21:24): "Eye for eye, tooth for tooth, hand for hand, foot for foot."
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 (61, 1; 64, 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.
Reply to Objection 1. Nothing prevents that which is contrary to a particular nature from being in harmony with universal nature: thus death and corruption, in the physical order, are contrary to the particular nature of the thing corrupted, although they are in keeping with universal nature. On like manner to maim anyone, though contrary to the particular nature of the body of the person maimed, is nevertheless in keeping with natural reason in relation to the common good.
Reply to Objection 2. The life of the entire man is not directed to something belonging to man; on the contrary whatever belongs to man is directed to his life. Hence in no case does it pertain to a person to take anyone's life, except to the public authority to whom is entrusted the procuring of the common good. But the removal of a member can be directed to the good of one man, and consequently in certain cases can pertain to him.
Reply to Objection 3. A member should not be removed for the sake of the bodily health of the whole, unless otherwise nothing can be done to further the good of the whole. Now it is always possible to further one's spiritual welfare otherwise than by cutting off a member, because sin is always subject to the will: and consequently in no case is it allowable to maim oneself, even to avoid any sin whatever. Hence Chrysostom, in his exposition on Matthew 19:12 (Hom. lxii in Matth.), "There are eunuchs who have made themselves eunuchs for the kingdom of heaven," says: "Not by maiming themselves, but by destroying evil thoughts, for a man is accursed who maims himself, since they are murderers who do such things." And further on he says: "Nor is lust tamed thereby, on the contrary it becomes more importunate, for the seed springs in us from other sources, and chiefly from an incontinent purpose and a careless mind: and temptation is curbed not so much by cutting off a member as by curbing one's thoughts."
Source: Summa Theologica (New Advent)