In this paper I revisit Gregory Kavka’s Toxin Puzzle and propose a novel solution to it. Like some previous accounts, mine postulates a tight link between intentions and reasons but, unlike them, in my account these are motivating rather than normative reasons, i.e. reasons that explain (rather than justify) the intended action. I argue that sensitivity to the absence of possible motivational explanations for the intended action is constitutive of deliberation-based intentions. Since ordinary rational agents display this sensitivity, when placed (...) in the toxin scenario they will believe that there is no motivational explanation for actually drinking the toxin and this is why they can’t form the intention to drink it in the first place. I thus argue that my Motivating-Explanatory Reason Principle correctly explains the toxin puzzle, thereby revealing itself as a genuine metaphysical constraint on intentions. I also explore at length the implications of my account for the nature of intention and rational agency. (shrink)