There is an explanatory constraint on practical reasons: practical reasons have to be the kinds of things that we can act for. Some philosophers, notably Bernard Williams, have argued that the explanatory constraint favours internalism about reasons: for an agent to have a reason to x, it is at least a necessary condition that she would, after ideal deliberation, be motivated to x. Internalism suggests that naturalism about reasons is more plausible for, in this view, reasons are psychological states. However, (...) reasons must not only be the kinds of things we can act for, at least some reasons must be good reasons, i.e. normative requirements. Derek Parfit argues that internalist, naturalist accounts cannot capture the normative aspect of reasons. He takes this to be a point in favour of his own externalist, non-naturalist account. I ask first whether Williams' account does fail to account for the normativity of reasons as Parfit argues, and second whether Parfit's externalist account can adequately meet the explanatory constraint. Although I claim that the answer to the first question is yes and the second no, the answer to both questions turns on the nature of practical rationality. My case stands if practical rationality is procedural, and falls if practical rationality is substantive. I argue, however, that the burden of proof is on those who, like Parfit, wish to defend a substantive account of practical rationality. S. Afr. J. Philos. Vol.24(2) 2005: 97-108. (shrink)