There are at least two basic normative notions: rationality and reasons. The dominant normative account of reasons nowadays, which I call primitive pluralism about reasons, holds that some reasons are normatively basic and there is no underlying normative explanation of them in terms of other normative notions. Kantian constructivism about reasons, understood as a normative rather than a metaethical view, holds that rationality is the primitive normative notion that picks out which non-normative facts are reasons for what and explains why (...) those normative relations hold. By supposing that there is a plurality of primitive reasons, I argue that primitive pluralism about reasons lacks sufficient normative unity and structure. But Kantian constructivism about reasons faces a dilemma of its own: Either a conception of rationality is thick enough to capture the reasons of commonsense, in which case it cannot play the explanatory role assigned to it, or a conception of rationality is genuinely explanatory, in which case it is too thin to generate the reasons we recognize in commonsense. The aim of this paper is to suggest that if Kantian constructivism about reasons were built on a substantive, rather than merely formal, conception of rationality then it would stand a better chance at unifying the particular reasons we would endorse on due reflection. The groundwork I lay in this paper, I explain, is an essential first step in the larger project of developing a version of Kantian constructivism about reasons that might eventually explain all reasons in terms of rationality. (shrink)