The Chicago tradition of Law and Economics confuses the principle of optimization with that of rationality. The paper attempts to show why this is the case, and to study the effects of this confusion on legal thinking. In the first part, it is argued that the rationality of the law does not manifest itself in the field of choice. The judge must apply the law. By contrast, the principle of optimization pretends that the concept of duty has no meaning independent of that of choice. However, this is neither philosophically nor logically true because, on the one hand, beside the ethic of responsibility there is the ethic of conviction; and on the other, we cannot explain duty by the logic of mathematics. The second part of the paper analyses the effects of the irreducibility of the concept of duty to that of choice, on the judge's decision. First of all, it is argued that the ethic of responsibility and the ethic of conviction describe the " normativity " of the law. The Chicago tradition of " Law and Economics " is founded exclusively upon the ethic of responsibility. The most important implication of this economic " normativity " is that the efficacy of the law can only be assessed on the basis of the effects that it tends to bring about, thus, forgetting that the application of moral rules is a necessity. Secondly, it will be argued that the Chicago tradition of " Law and Economics " is characterized by a paradox. On the one hand, this tradition is based on the ethic of responsibility. On the other hand, however, the individual who commits a crime is not considered to be responsible for his act, which he committed because the penalty was too low. Thirdly, it is argued that it is difficult to judge the action without knowing the agent's purpose. Did the individual kill be accident or intentionally? According to the optimization principle, an action can be judged only by its effects. All acts are purposeful. We argue that this is not true. Thus the Chicago tradition of " Law and Economics " has to develop a theory of action within the framework of the concept of " duty ".