Practical reason

Practical reason is the use of reason to decide how to act, that is, to determine what ought to be done in a concrete situation. It is contrasted with theoretical reason, which concerns what to believe or what is true, and with productive or technical reason, which seeks the best means to a given end. Where theoretical reason answers the question of what is the case, practical reason answers the question of what should be done.

The notion goes back to classical Greek philosophy, where Aristotle distinguished theoretical, practical and productive knowledge, and gave pride of place to philosophical activity. In the Thomistic tradition, the first principle of practical reason is stated as: good is to be done and pursued, and evil is to be avoided. For Kant, practical reason has a legislative character: the categorical imperative binds the subject through duty rather than subjective inclinations. Utilitarians, by contrast, tend to understand reason as an instrument serving the satisfaction of wants and needs. John Finnis, within the natural law tradition, holds that the first principles of action are grasped immediately by the intellect and cannot be deduced from what is.

A simple example: deciding whether to build a telescope is a matter of practical reason, whereas determining which of two theories of light is more accurate belongs to theoretical reason. In practical reasoning, an agent starts from a goal, selects an action as a means to reach it, and arrives at a reasoned decision to carry it out. Argumentation theory distinguishes instrumental practical reasoning, which does not explicitly take values into account, from value-based practical reasoning.

The debate remains open: whether practical reasoning is purely instrumental or must rest on values, and to what extent reason can judge ends and not merely means. Since Descartes, practical judgment has been treated with less respect in the face of the demand for certainty and an infallible method. Today the concept is central also in artificial intelligence, law, medicine and engineering, where autonomous reasoning about which actions to take is required.