Kant has preached that lying is forbidden and that always speaking the truth is a categorical imperative (moral law). In his essay, “Truthfulness and Lies: What Can We Learn from Kant?” Alasdair MacIntyre examines Kant’s notion of radical truthfulness and notes that speaking half-truths in difficult circumstances is allowed under the Kantian moral system.
Kant himself used a half-truth to outmaneuver King Friedrich Wilhelm II and the royal censors who were accusing him of disparaging religion in his writings. The censors demanded that Kant should make the pledge that henceforth he would refrain from writing on religion. To assuage the King, Kant made this statement: “As your Majesty’s faithful subject, I shall in the future completely desist from all public lectures or papers concerning religion.”
MacIntyre points out that Kant has claimed that when he made the statement to the King, he knew that the King was old and frail and not likely to live for too long. When the King died, Kant regarded himself as free of the pledge, since his pledge was made “as your Majesty’s faithful subject” and was valid so long as the Majesty was alive. Thus, Kant was free to resume writing on religious issues.