Reductio Ad Absurdum argument, from a dialog with ifagent 
( http://subbot.org/ifagent/ifagent.txt ):

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> Assume animals can argue.
Okay, I will assume animals can argue.

> If animals can argue, then they are intelligent.
Okay, animals can argue -> they are intelligent.

> If animals are intelligent, then we shouldn't eat or own them.
Okay, animals are intelligent -> we shouldn't eat or own them.

> But "we shouldn't eat or own them" is absurd! They are delicious, and make such good pets!
Therefore, animals can not argue.
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