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Practical Reason is the rational capacity by which (rational) agents guide their conduct.

Practical reason decides how the world should be and what individuals should do. Practical reason decides what to do, it cannot remake reality any way it likes. How individuals ought to believe is then a practical question. Some of the interest in practical reason comes from trying to understand its failures.

The successful practical agent must take into account truths about the world. Some have inferred from this that practical reason consists largely (or entirely) in using such knowledge for practical purposes.

Practical reason is the application of theoretical reasoning and its conclusions to concrete, practical situations.  Practical reasoning is instrumental, calculating how to achieve an end that is not itself rationally determined. To be practically rational is intelligently to pursue one’s interest. “Reason is, and ought only to be, the slave of the passions”, and it is not irrational to “prefer the destruction of the whole world to the scratching of my finger”.

If reason is a slave of the passions, then practical reasoning is simply the name for reasoning that concerns itself with desires and the means of satisfying them.

Practical reasoning does not differ from theoretical reasoning except in content: it is reasoning that is about preferences, desires, obstacles, and resources. If good people do not teach virtue, it is simply because they choose not to pass on their cleverness; virtue is taught by conditioning or persuading people to desire and take pleasure in the right things.

Practical reason understands laws of freedom. For Kant, practical reason has priority because the knowledge of theoretical reason is only knowledge of phenomena—how things appear to us—while practical reason orients itself to things as they really are. Moreover, even theoretical reason depends on practical reason, since Kant insists that “reason has no dictatorial authority; its verdict is always simply the agreement of free citizens”.

For Kant Practical Reason cannot take some existing practices of practical reason as data to be explained. It is only by transcending the empirical conditions that limit practical reason to an instrumental role that one discovers, in pure practical reason, the legitimate moral employment of rationality. It is known that desires cause actions. Kant asks whether reason can lead to action on its own, and not only concludes that it can, but argues that actions caused by reason alone are identical with morally good acts.

In Immanuel Kant’s Moral Philosophy, Practical Reason is defined as the capacity of a rational being to act according to principles (i.e., according to the conception of laws). Unlike the ethical intuitionists (see intuitionism), Kant never held that practical reason intuits the rightness of particular actions or moral principles. For him, practical reason was basically formal rather than material, a framework of formative principles rather than a source of specific rules.

Kant claims to have discovered the supreme principle of practical reason, which he calls the Categorical Imperative. Notoriously, Kant offers several different formulations of this principle, the first of which runs as follows: “act only in accordance with that maxim through which you can at the same time will that it become a universal law”. Kant holds this principle to be implicit in common human reason: when we make moral judgments, we rely on this criterion, although invariably we do not articulate it as such.


The Essential Dialectic of Practical Reason is:

{Categorical-Intellectual ⇆ Intellectual-Categorical ⇅ Categorical-Categorical} ↻ Intellectual-Intellectual


The Complete Dialectic of Practical Reason is:

{Intellectual-Imperative ⇆ Categorical-Imperative ⇅ Formal-Principle} ↻ Will



The Equivalency Dialectic of Practical Reason is:

{Speculative ⇆ Theoretical ⇅ Practical} ↻ Judicial