It would seem that the ingratitude, whereby a subsequent sin causes the return of sins previously forgiven, is a special sin. For, the giving of thanks belongs to counter passion, which is a necessary condition of justice. But justice is a special virtue. Therefore this ingratitude is a special sin. Thanksgiving is a special virtue. But ingratitude is opposed to thanksgiving. Therefore ingratitude is a special sin.
Thomas AquinasBehold our refutation of the error. It is not based on documents of faith, but on the reasons and statements of the philosophers themselves. If then anyone there be who, boastfully taking pride in his supposed wisdom, wishes to challenge what we have written, let him not do it in some corner nor before children who are powerless to decide on such difficult matters. Let him reply openly if he dare. He shall find me there confronting him, and not only my negligible self, but many another whose study is truth.
Thomas AquinasEvery judgement of conscience, be it right or wrong, be it about things evil in themselves or morally indifferent, is obligatory, in such wise that he who acts against his conscience always sins.
Thomas AquinasIt is altogether unlawful to kill oneself... Wherefore suicide is contrary to the inclination of nature, and to charity whereby every man should love himself... Life is God's gift to man, and is subject to His power, Who kills and makes to live. Hence whoever takes his own life, sins against God... for it belongs to God alone to pronounce sentence of death and life.
Thomas Aquinas