Introduction

The problem of the existence of a pure reason capable of determining the will, that is, capable of being practical, is an issue for philosophy of action, which is paramount to ethics since any discussion on whether an action is good or bad or on what must be done has to presuppose that action is possible at all. To deny the existence of a pure practical reason seems to imply the denial of the very possibility of making an agent accountable for his or her actions and, therefore, the very purpose of philosophical matters such as ethics, political philosophy, philosophy of law, etc. At least, it seems to deny that it is possible to act under rules capable of being objective, that is, universally valid.

In the history of philosophy, there were antagonistic views on this question. For instance, Hume held that reason is only capable of distinguishing what is true from what is false and it is unable to determine the will. In his own words: “Reason is the discovery of truth or falsehood. … Reason is wholly inactive.” To the author of A Treatise of Human Nature, reason is a slave of passions, that is, the ends of actions are established by the will and reason is only capable of saying whether a means x is better or worse than another means y to reach the desired end.