devil's advocate

1. One who argues against a cause or position, not as a committed opponent but simply for the sake of argument or to determine the validity of the cause or position. \
2. Roman Catholic Church. An official appointed to present arguments against a proposed canonization or beatification.

dialectics

A method of argument or exposition that systematically weighs contradictory facts or ideas with a view to the resolution of their real or apparent contradictions.

American Heritage Dictionary