Abstract: Reasoning is generally seen as a mean to improve knowledge and make better decisions. Much evidence, however, shows that reasoning often leads to epistemic distortions and poor decisions. This suggests rethinking the function of reasoning. Our hypothesis is that the function of reasoning is argumentative. It is to devise and evaluate arguments intended to persuade. Reasoning so conceived is adaptive given human exceptional dependence on communication and vulnerability to misinformation. A wide range of evidence in the psychology or reasoning and decision making can be reinterpreted and better explained in the light of this hypothesis.