Check out the Wikipedia page of "truth": This statement is true. "This statement is true" is true. "'This statement is true' is true" is true. And so on and so forth... the iterations are always true. On the other hand, "This statement is false" leads to "This statement is false is true" and further "'This statement is false' is true" is false". Therefore, as you can see, it is easy to deal with what is true but very tricky to deal with what is false. Same with honest and dishonest. When a person says "I am lying", he is telling the truth that he is lying but if you believe him, that it is true that he is lying, then he is lying when he is telling the truth that he is lying, and so on, and so forth. Now tell me, how can you "always trust a dishonest man to be dishonest"?