Prev   -   Next Search:

 Conditional Any two propositions P and Q can be joined by a conditional operator, producing the new, complex, proposition: If P then Q The proposition If P then Q is true if and only if either P is false or Q is true. It is false only when P is true and Q is false. The truth table for P or Q is as follows: If you have Java, try it out for yourself: Set the truth values for the propositions P and Q by clicking on the appropriate button beside "P" and "Q" respectively. What is the truth value of If P then Q? Find out by clicking on the "Compute" button. P is: T F Q is: T F Thus, If P then Q is: A special conditional occurs if we flip the P and Q around: we get if Q then P, which is the same as saying P only if Q. What is the truth value of P only if Q? Find out by clicking on the "Compute" button. P is: T F Q is: T F Thus, P only if Q is: Notice that P only if Q is false only when Q is true and P is false. 17 August 1996 For Educators Stephen Downes Guide to the Logical Fallacies Copyright © Stephen Downes, 1995-2001 stephen.downes@ualberta.ca