Pragmatic presuppositions are analyzed considering their relation with the notion of commitment, namely the dialogical acceptance of a proposition by an interlocutor. The attribution of commitments carried out by means of pragmatic presupposition is shown to depend on the reasonableness of the underlying presumptive reasoning, ultimately grounded on hierarchies of presumptions. On this perspective, the ordinary interpretation of pragmatic presuppositions as the "taking for granted" of propositions signaled by semantic or syntactic triggers becomes only the presumptive, prototypical interpretation of a complex linguistic and pragmatic phenomenon. It will be shown how the prototypical interpretation is subject to default in cases of conflicts of presumptions, which lead to reconstructing the speaker's meaning non-presumptively at a pragmatic, semantic, or syntactic level. The phenomena of presupposition cancellation and neutralization can be explained in terms of presumptive and non-presumptive articulation and interpretation of an utterance, through which the speaker can impose, correct, or refuse implicit commitments.