There are at least three basic phenomena that philosophers traditionally classify as paradigm cases of irrationality. In the first two cases, wishful thinking and self-deception, a person wants something to be true and therefore ignores certain relevant facts about the situation, making it appear to herself that it is, in fact, true. The third case, weakness of will, involves a person undertaking a certain action, despite taking herself to have an all-things-considered better reason not to do so.
While I think that Stephen Colbert's notion of "truthiness" might be able to fit the mold of each of these three kinds of irrationality, it applies most directly to cases of wishful thinking and self-deception — and it’s these two types of irrationality that I discuss extensively in this paper. As we will see, there are some troubling philosophical problems that arise regarding phenomena like self-deception. But we can use the concept of truthiness to show how these “paradoxes of irrationality” may be resolved without denying the fundamental irrationality of truthiness itself.
An earlier version of this paper ("Truthiness, Self-Deception, and Intuitive Knowledge") appeared in "The Daily Show and Philosophy" (2007).