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virtue of its encyclopedic representation, a complex frame of reference, the sememe can be seen as a virtual text (Eco 1979a).
In the case of presuppositions, we call positional power the power that presuppositional expressions carry to impose on the context of discourse a given semantic content (their presuppositional content). This means that the Addressee will assume presuppositions as part of a shared background: what the Addressee doesas soon as a p-term or a p-construction is inserted into the discourseis to contextualize the expression in the appropriate context, which means to create such a context if it is not given. The appropriate context is, of course, a context where presuppositions are compatible with other information, that is, are assumed as an unchallenged background. On the other hand, the Speaker uses a presuppositional expression to make the Addressee believe in that background frame. Such a semantic frame is encoded in the language systemby virtue of semantic organization in the case of p-terms and by virtue of grammatical form in other casesand it can be accounted for in the semantic representation.
This does not mean that presuppositions are unchallengeable; given certain contextual conditions they can be cancelled, and in this case the positional power will not completely coincide with the presuppositional power represented in the semantic system. However, in order to challenge presuppositions, some particular rhetoric strategy is required: A has to challenge the right of S to use the expression S used, then employing a metalinguistic negation. Therefore, presuppositional terms and sentences can only be negated de dicto, never de re. De dicto negation affects textual organization, as we will see in the following section.
1.3. Challenging Presuppositions
We have said that presuppositions can. be denied. The problem of presupposition negation has been much discussed in recent literature on presuppositions. For some authors, who try to reduce presupposition relation to entailment, the possibility of denying presuppositions is considered an argument against their existence, and a challenge to the validity of the very notion. Kempson (1975) claims that a sentence such as (8) does not have presuppositions:
(8) Edward didn't regret that Margaret had failed because he knew it wasn't true.
It is clear that there is an "intuitive" difference between (8) and (9), a difference which cannot be explained in Kempson's analysis:

 
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