Every 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.
Saint Thomas Aquinas (1225 – 7 March 1274), was an Italian Dominican friar, Catholic priest, and Doctor of the Church. He was an immensely influential philosopher, theologian, and jurist in the tradition of scholasticism, within which he is also known as the Doctor Angelicus and the Doctor Communis.
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