The observation that humans are often irrational has become commonplace. This observation has received empirical support from various experiments performed by cognitive scientists that are supposed to show that humans systematically violate principles of probability, rules of logic, and other norms of reasoning. In response to these experiments, philosophers have made creative and appealing arguments that these experiments must be mistaken or misinterpreted because humans must be rational. I examine these arguments for human rationality and show that they fail; cognitive (...) science does play a role in assessing human rationality. ;In Chapter One, I examine three cognitive science experiments in detail and discuss what is at issue in the debate about human rationality. At issue is whether the mistakes humans make are mere mistakes or indicative of systematic divergences from norms of reasoning. Defenders of human rationality say these mistakes are not characteristic of underlying reasoning abilities but are performance errors. Because humans make performance errors, human reasoning ability cannot be read off of behavior, but rather is to be studied by exploring cognitive competence, the underlying ability to reason. ;In Chapter Two, I develop and discuss the notion of cognitive competence. I then consider and reject two arguments that human cognitive competence must match the norms of reasoning. The first argument appeals to simplicity considerations, the second to the supposed common origin of our cognitive competence and our normative standards. In Chapter Three, I turn to a related argument for a view that humans are rational that says cognitive competence is discovered in the same way norms of reasoning are justified: both projects involve reflective equilibrium using as data people's judgments about what counts as good reasoning. ;In Chapter Four, I consider an argument for human rationality that draws on the principle of charity: when translating utterances of a speaker, one should try to interpret her utterances as rational and true. ;In Chapter Five, I consider an argument for human rationality that appeals to evolution. This argument claims that natural selection will select for cognitive mechanisms that produce true beliefs, and that such "truth-tropic" mechanisms are sufficient for being rational. ;My overall conclusion is that while cognitive science does not determine what the normative principles of reasoning are, it is relevant to whether or not human cognitive competence matches these principles; whether or not humans are irrational is, thus, an empirical question. (shrink)