This is an age of naturalization projects. Much epistemological work has been done toward naturalizing theoretical reason. One might view Hume as seeking to naturalize reason in both the theoretical (roughly, epistemological) and the practical realms. I suggest that whatever else underlies the vitality of Hume's instrumentalism - encapsulated in his view that 'reason is and ought only to be the slave of the passions' - one incentive is the hope of naturalizing practical reason. This paper explores some broadly Humean versions of instrumentalism that are among the most plausible contenders to represent instrumentalism as a contemporary naturalistic position. It first offers a taxonomy of reasons for action and, in that light, formulates a plausible version of instrumentalism. It then raises difficulties for the view, some of them concerning the nature of desire. It also develops an epistemologically significant comparison of desires with beliefs. Given the magnitude of the difficulties, it outlines an alternative account of practical reason.