Apagoge

• (n.) An indirect argument which proves a thing by showing the impossibility or absurdity of the contrary.
Found on http://thinkexist.com/dictionary/meaning/apagoge/

Apagoge

(Gr. apagoge) In Aristotle's logic (1) a syllogism whose major premiss is certain but whose minor premiss is only probable; abduction; (2) a method of indirect demonstration whereby the validity of a conclusion is established by assuming its contradictory and showing that impossible or unacceptable consequences follow; the reductio ad impossibile.....
Found on http://www.ditext.com/runes/a.html

apagoge

proof by showing the falsehood of the opposite
Found on http://phrontistery.info/a.html
No exact match found