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===Demonstração por absurdo===

===Demonstração por absurdo===

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.

Além disso, na lógica clássica, existe um princípio denominado <u> terceiro excluído </u>, que declara que uma frase que não pode ser falsa deve ser considerada verdadeira, uma vez que não existe uma terceira possibilidade.

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