Pragmatic presupposition is analyzed as grounded on an implicit reasoning process based on a set of presumptions, which can define cultural differences. The basic condition for presupposing a proposition is represented as a reasoning criterion, namely reasonableness. Presuppositions, on this view, need to be reasonable, namely they need to be the reasonable conclusions of an underlying presumptive reasoning, which does not or may not contain contradictions with other presumptions. Presumptions are in turn analyzed considering their nature and their hierarchy, namely their subject matter and their possible contextual backing, which eliminates some of their possible defaults. This analysis of presupposition brings to light the relationship between misunderstandings deriving from presuppositional failures and the underlying system of presumptions. This approach is applied to the investigation of communicative issues within the medical context, and more precisely doctor-patient communication in diabetes cases.