The idea that reasoning can be implicit, or that there are implicit inferences involved in reasoning, is as plausible as it is problematic. On the one hand, there are good reasons to think that not all (or even most) of the inferential steps in reasoning are explicit. On the other hand, the idea that there is implicit reasoning might be taken to embody an intellectualist or rationalist myth about how the mind really works that needs to be replaced by better accounts. These conflicting tendencies have a long history at the intersection between philosophy and psychology. We develop our argument by describing and evaluating some prominent accounts of the ways in which the notion of implicitness as applied to reasoning has been analyzed in the history of philosophy and psychology. Addressing views from Aristotle over Hume, Kant and Helmholtz up to current dual systems accounts such as Daniel Kahneman’s, we discuss and clarify the meanings of ‘implicit’ and ‘reasoning’. We show that the non-conscious and automatic nature of some cognitive processes does not preclude them from being reasoning processes, nor from being responsive to norms of rationality. Some implicit processes can indeed be instances of reasoning: because they can be made explicit and then also be evaluated critically just like standard, conscious or deliberate instances of reasoning.