Book excerpt: The scientific area this thesis belongs to is many-valued logics: this meanslogics 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 thatevery 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 suggestto reject this law: for example, intuitionistic logic does not satisfy it, sincethis logic reflects a "constructive" conception of mathematics (see [Hey71, Tro69]).