Full Idea
In the traditional account, a sentence s presupposes p if and only if both s and Žs entail p. Standardly, this entails that if s presupposes p, then whenever p is false, s must be neither true nor false.
Gist of Idea
If both s and not-s entail a sentence p, then p is a presupposition
Source
Ofra Magidor (Category Mistakes [2013], 5.3.2)
Book Reference
Magidor,Ofra: 'Category Mistakes' [OUP 2013], p.124
A Reaction
'I'm looking down on the garden' presupposes 'I'm upstairs'. Why would 'I'm not looking down on the garden' entail 'I'm upstairs'? I seem to have missed something.