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===Dimostrazione per assurdo===

===Dimostrazione per assurdo===

Furthermore, in classical logic there is a principle called the <u>excluded third</u> which declares that a sentence that cannot be false must be taken as true since there is no third possibility.

Inoltre, nella logica classica c'è un principio chiamato il <u>terzo escluso</u> che dichiara che una frase che non può essere falsa deve essere presa come vera poiché non c'è una terza possibilità.

Suppose we need to prove that the proposition <math>p</math> is true. The procedure consists in showing that the assumption that <math>p</math> is false leads to a logical contradiction. Thus the proposition <math>p</math> cannot be false, and therefore, according to the law of the excluded third, it must be true. This method of demonstration is called ''demonstration by absurdity''<ref>{{Cite book  

Suppose we need to prove that the proposition <math>p</math> is true. The procedure consists in showing that the assumption that <math>p</math> is false leads to a logical contradiction. Thus the proposition <math>p</math> cannot be false, and therefore, according to the law of the excluded third, it must be true. This method of demonstration is called ''demonstration by absurdity''<ref>{{Cite book