This paper investigates the norm of presupposition, as one pervasive type of indirect speech act. It argues against the view that sees presuppositions as an indirect counterpart of the direct speech act of assertion and proposes instead that they are much more similar to the direct speech act of assumption. More concretely, it suggests that the norm that governs presuppositions is not an epistemic or doxastic attitude such as knowledge, justified belief, or mere belief; it's a practical attitude, most plausibly (...) the attitude of rational acceptance. This view has important ramifications well beyond debates in philosophy of language and linguistics. It affects not only our view of which speech act sequences are fine and which are off; it bears on whether presuppositions can function as testimony, whether they can be lies, and whether they are ontologically committal, thus addressing central topics in epistemology, ethics, and metaphysics. (shrink)