The scientific area this thesis belongs to is many-valued logics: this means logics in which, from the semantical point of view, we have “intermediate” truth-values, between 0 and 1 (which in turns are designated to represent, respectively, the “false” and the “true”).The classical logic (propositional, for simplicity) is based on the fact that every statement is true or false: this is reflected by the excluded middle law, that is a theorem of this logic. However, there are many reasons that suggest to reject this law: for example, intuitionistic logic does not satisfy it, since this logic reflects a “constructive” conception of mathematics (see [Hey71, Tro69]).

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