What is it that one fundamentally rejects when one criticizes a way of thinking as moralistic? Taking my cue from the principal leveller of this charge in philosophy, I argue that the root problem of moralism is the dualism that underlies it. I begin by distinguishing the rejection of moralism from the rejection of the moral/nonmoral distinction: far from being something one should jettison along with moralism, that distinction is something that any human society is bound to develop. But this valuable distinction is transformed into a problematic dualism when it casts the two sides of the distinction as contrasting sharply in nature, value, and structure. In ethics, the resulting dichotomy takes the form of a dualism of morality and prudence. In politics, it takes the form of a dualism of principle and interest. I explain the enduring appeal of such dualisms before laying out the costs of moralism thus conceived: moralism erodes our sensibility to the moral and political costs of value conflicts; it projects an unrealistic conception of agency that sets up scepticism about responsibility; and it limits our ability to appreciate and realize the wider variety of nonmoral values that sustain us, our achievements, and morality itself.