Kant and Sidgwick are at opposite extremes on whether we may tell paternalistic lies. I trace the extremism to their views about ethical concepts. Sidgwick thinks fundamental ethical concepts must be precise. Common Sense morality says we may tell paternalistic lies to children but not to sane adults. Because the distinction between a child and an adult is imprecise, Sidgwick thinks this principle cannot be fundamental, and must be based on the {precise) principle of utility, which often mandates paternalistic lies (...) to adults. Kant thinks that ethical concepts are ideals of reason, which do not fit the world precisely because the world is imperfect. We lie to children and the insane because they are irrational, but no one is perfectly rational. We must treat all persons with the respect due to rational agents, so the pressure of the theory is toward not lying to anyone. Decisions about where to draw the line must be made pragmatically and to some extent arbitrary. But fear of this is not a good reason to abandon ethical ideals for utilitarianism. (shrink)