a statement that is necessarily true (such as "She will win the election or she won`t.")
She didn’t proved anything. It’s a tautology to propose as a premise that "All men are pigs," and that "Bob is therefore a pig because he’s a man." The "proof" is in the premise—which is to say, there is no proof.
As a logical proposition, that paragraph can be reduced to the following: Because A, therefore A—the sort of tautology that would get any high school student a failing grade.
Michael Elliott -- Time Inc., "How to Lose Friends and Alienate People" (Dec. 22, 2003) -- http://www.time.com/time/printout/0,8816,1006489,00.html(retrieved 09/24/08)
...the definition of ’self-interest’ as the pursuit of the fulfillment of preferences is a tautology.