With his new book "Intuition Pumps and Other Tools for Thinking," (http://goo.gl/FtKWl) philosopher Daniel Dennett offers a kind of self-help book for deep thinkers -- a series of thought experiments designed as a workout for the deliberative mind. Here he discusses reductio ad absurdum, "the workhorse of philosophical argumentation," wherewith thinkers test the validity of an opponent's argument by taking it to its most illogical extreme.