 For example, it is objectively right—in accordance with the natural law—to refrain from sex outside of marriage. But suppose that an agent (mistakenly) believes that such abstinence is sinful, and yet still refrains. According to Aquinas, this agent, although he does what is objectively right, nonetheless sins. Why? Because in choosing the opposite of what his conscience dictated, he chose to do something he believed to be evil. And choosing what is (subjectively if not objectively) wrong, for Aquinas, is a sin: “[F] from the very fact that a thing is proposed by the reason as being evil, the will by tending thereto becomes evil” (Summa theologica, I-II, q. 19, a. 5, corp.).