Keelefilosoofia raamat
"
A few philosophers and many more linguists took up Strawson's idea, and
have made it a little more formal: When a sentence S1 entails a sentence S2 ,
and S2 is false, then necessarily S1 is false and S1's negation is true. But when
S1 presupposes S2 and S2 is false, then S1 does not go false, but lacks truth-
value, and so does S1's negation.5 Notice that presupposition in this sense
(called semantic presupposition) is like entailment and unlike conversational
implicature in being noncancelable. Both S1 and S1's negation necessitate S2 in
the absolute way characteristic of entailment.
There are actually no uncontroversial examples of semantic presupposi-
tion. But here are a few candidate sentence pairs.
(7a) It was Grannie who robbed the diaper service.
(7b) Someone robbed the diaper service.
(8a) Have you stopped beating your spouse?
(8b) You have beaten your spouse.
(9a) Rocky realized that his fly was open.
(9b) Rocky's fly was open.
annaabi.ee 
