This chapter argues that the most important issue for the cognitive science of reasoning is whether deduction provides a computational-level theory of a substantial amount of everyday, commonsense thought. The chapter is organized as follows. It begins by outlining what deduction is, in abstract terms, and then considers various ways in which it can be related to human reasoning, using the framework of D. Marr's levels of explanation. Three sophisticated lines of arguments are developed from epistemology, AI, and the psychology of reasoning. Each argument supports the conclusion that deduction has no significant role in commonsense reasoning. The implications of rejecting deduction for the cognitive science of human reasoning are considered.
Keywords: D. Marr, cognitive science, deduction, computational-level theory, commonsense reasoning, epistemology, AI, reasoning