In this paper, the author considers an argument against the thesis that humans are irrational in the sense that we reason according to principles that differ from those we ought to follow. The argument begins by noting that if humans are irrational, we should not trust the results of our reasoning processes. If we are justified in believing that humans are irrational, then, since this belief results from a reasoning process, we should not accept this belief. The claim that humans are irrational is, thus, self-undermining. The author shows that this argument and others like it fail for several interesting reasons. In fact, there is nothing self-undermining about the claim that humans are irrational; empirical research to establish this claim does not face the sorts of a priori problems that some philosophers and psychologists have claimed it does.