In this paper I trace the development of one of the central debates of late twentieth-century moral philosophy—the debate between realism and what Rawls called “constructivism.” Realism, I argue, is a reactive position that arises in response to almost every attempt to give a substantive explanation of morality. It results from the realist’s belief that such explanations inevitably reduce moral phenomena to natural phenomena. I trace this belief, and the essence of realism, to a view about the nature of concepts—that (...) it is the function of all concepts to describe reality. Constructivism may be understood as the alternative view that a normative concept refers schematically to the solution to a practical problem. A constructivist account of a concept, unlike a traditional analysis, is an attemptto work out the solution to that problem. I explain how the philosophies of Kant and Rawls can be understood on this model. (shrink)