philosophers use logic to solve problems and answer questions. logical consistency is a cornerstone of any acceptable theory. philosophers who disagree with a theory will often try to find a logical contradiction in a theory. if they find a contradiction, this gives them a reason to reject that theory. if they do not find an inconsistency, the philosopher might show that the theory leads to a conclusion which is either unacceptable or ridiculous. this second approach is called reductio ad absurdum.