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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.

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.

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>Luıs Moniz Pereira and Alexandre Miguel Pinto. [http://www-lia.deis.unibo.it/confs/ArgNMR/proceedings/ArgNMR-proceedings.pdf#page=100 Reductio ad Absurdum Argumentation in Normal Logic Programs]</ref>''

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

| autore = Pereira LM

| autore2 = Pinto AM

| url = http://www-lia.deis.unibo.it/confs/ArgNMR/proceedings/ArgNMR-proceedings.pdf#page=100

| volume = Argumentation and Non-Monotonic Reasoning - An LPNMR Workshop

}}</ref>''

=== Predicates ===

=== Predicates ===