Imagine that a depraved tyrant is oppressing your country, driving its citizens into misery. You have the opportunity to save the country: By starting an affair with the tyrant’s wife, you can gain access to the tyrant and depose him. Is that OK? This case was raised by a commentator on Aristotle, who thought it a circumstance in which adultery might be justified. But St. Thomas Aquinas, in his De Malo, says that the commentator is wrong. “One ought not to commit adultery for any benefit,” St. Thomas writes, expressing the constant teaching of the Church. Some acts, whatever the...