This dissertation focuses on the massive modularity hypothesis defended by evolutionary psychologists---the hypothesis that the human mind is composed largely or perhaps even entirely of special purpose information processing organs or "modulees" that have been shaped by natural selection to handle the sorts of recurrent information processing problems that confronted our hunter-gatherer forebears. ;In discussing MMH, I have three central goals. First, I aim to clarify the hypothesis and develop theoretically useful notions of "module" and "domain-specificity" that can play the (...) roles required of them by evolutionary psychology. Second, I aim to evaluate the plausibility MMH in the light of the broad range of arguments that have been developed and defended in the literature. I argue that all the main, general arguments both for and against MMH are unsatisfactory. Moreover, I suggest that if the case for MMH is to be made, it will only result from the successive accumulation of specific, empirical evidence for the existence of particular modules. ;Finally, I address a range of issues that arise from evolutionary psychological approaches to reasoning and rationality. Much of what evolutionary psychologists have said about human reasoning is in response to a widely discussed "pessimistic" interpretation that has been developed and defended by Kahneman, Tversky and their followers. According to this view, human beings are prone to systematic deviations from appropriate norms of rationality because they lack the underlying competence to handle a wide array of reasoning tasks. Evolutionary psychologists appear to reject this pessimistic interpretation in favor of the view that we possess a wide range of reasoning modules that employ rational rules of inference. I argue, however, that there is no genuine disagreement between evolutionary psychologists and their opponents over the extent to which human beings are rational. I also discuss the distinction between competence efforts and performance efforts. Although this distinction has played a central role in recent discussions of human rationality, I argue that if MMH is true, then we face insurmountable problems in trying to draw the performance error/competence error distinction. (shrink)