This paper offers an original argument for a Humean thesis about promising that generalises to the domain of normative powers. The Humean ‘natural unintelligibility’ thesis – prominently endorsed by Rawls, Hart, and Anscombe, and roundly rejected or forgotten by contemporary writers (conventionalists and non – conventionalists alike) – holds that a rational, suitably informed agent cannot so much as make a promise (much less a morally-binding promise) without exploiting conventional norms that confer promissory significance on act types (e.g., signing on (...) the dotted line) that would not otherwise have such significance. The argument (like Hume’s) is action-theoretic in character; its central premise is the Contribution Condition on acting with an aim: roughly, that doing X in order to (bring about) Y requires that the agent believe that their having X-ed on the occasion might come to fully or partly account for Y’s coming to pass. The argument generalises to the broader domain of exercises of normative powers, i.e., to acts that modify normative properties by ‘fiat’ or ‘stipulation’. This stipulative act has been characterised in three different ways, each of which is given due consideration. Finally, two different ways of accommodating the ‘natural unintelligibility’ thesis are considered, as is the thesis’s normative significance. (shrink)