The main aim of this paper is to explore why it is so important for Hume to defi ne ‘cause’ as he does. This will shed light on the signifi cance of the natural/philosophical relation (hereafter NPR) distinction in the Treatise. Hume's use of the NPR distinction allows him to dismiss on general grounds conceptions of causation at odds with his own. In particular, it allows him to avoid having to engage in detailed re-interpretation of potentially confl icting theories formulated (...) by natural philosophers. Moreover, it provides an instance of the normative nature of Hume's “science of man.” The paper argues that the NPR distinction - in conjunction with the so-called copy principle - is meant to undercut appeals to the authority of theories not founded on Hume's “principles.” In order to illustrate its claims about Hume, this essay explores some aspects of Newton's natural philosophy. Finally, this paper resolves a long-standing interpretive problem: how to reconcile Hume's two “defi nitions” of causation in the Treatise. (shrink)