This text considers the contributions that cognitive sciences can make to the study of legal reasoning, distinguishing between descriptive, conceptual and normative impacts. In particular, it is concerned with exploring Jonathan Haidt’s social intuitionism thesis, which says that, when we reason about moral and practical issues, we make a decision intuitively, which we then rationalise a posteriori (although at this stage reason cannot change the decision made). The text considers how this thesis would apply to the problem of the defeasibility of rules, which it takes as one of the characteristic features of legal reasoning. Finally, some objections are presented to Haidt’s thesis and to the normative claims of some cognitive scientists.