Full Idea
According to the pragmatic approach, presuppositions are constraints on the context: if a sentence s generates a presupposition p, an assertion of s cannot proceed smoothly unless the context already entails p (p is taken for granted).
Gist of Idea
In the pragmatic approach, presuppositions are assumed in a context, for successful assertion
Source
Ofra Magidor (Category Mistakes [2013], 5.3.2)
Book Reference
Magidor,Ofra: 'Category Mistakes' [OUP 2013], p.124
A Reaction
She credits Stalnaker for this approach. There is a choice between the presuppositions being largely driven by internal features of the sentence, or by external features of context. You may not know the context of some statements.