In this paper I offer a partial defense of a constitutivist view according to which it is possible to defend fundamental requirements of practical reason by appeal to facts about what is constitutive of rational agency. I show how it is possible for that approach to circumvent the ‘is’/’ought’ problem as well as the requirement that it be possible to act contrary to practical reason. But I do not attempt to establish any particular fundamental requirement. The key ideas are that (...) such a requirement is not genuine if it is arbitrary, and that it is arbitrary just in case (a) it needs explanation and (b) that explanation could not, even in principle, be provided. (shrink)