Kant’s Practical Reason: Principles and Postulates

Posted on Mar 14, 2025 in Philosophy and ethics

Principles of Pure Practical Reason

At the beginning of the Critique of Practical Reason, Kant synthesizes its contents with a definition, four theorems, and a fundamental law:

Definition: “Practical principles are propositions that contain a universal determination of the will, which are subordinated to several rules of thumb. They are subjective, or maxims, when the condition is considered by the subject as valid only for his will. They are objective, or practical laws, when the condition is valid for the will of every rational being.”

Fundamental Law of Pure Practical Reason: “Act so that the maxim of your will can always hold at the same time as a principle of universal legislation.”

Formal Morality

Under what conditions can a practical principle be asserted as law?

  • Practical or moral law must be strict; that is, necessary and universal.
  • Therefore, for a principle to be cited as having universal validity, it cannot be a maxim.

From this, two types of morality emerge:

  • Material Morality (Theorems I and II): Its principles are materials that determine the will by the objects offered by desire, and ultimately, always propose the search for happiness. Universal material morals only contain maxims, not laws.
  • Formal Law (Theorems III and IV): Regardless of what the will can be determined by, only the form, its universality, remains. The law states: “Act so that your maxim could be a law for all,” meaning your behavior can become a law for everyone. This is the formula of the categorical imperative, an imperative valid a priori in itself. The categorical imperative expresses the necessary and sufficient determination of the will and is unique. In the Groundwork, Kant includes various formulations: universal law, autonomy of the will, and end in itself.

Freedom and Autonomy of Morality

If the law is a fact that need not be deduced, the existence of the law can be deduced from liberty. This freedom has two dimensions:

  • Negative: Freedom is not conditioned by anything empirical.
  • Positive: Autonomy, meaning to be free and to be subject to a moral law, coincide.

Right and Duty

It is the concept of good that underlies moral law, but conversely, it is the law that determines what is good – a Copernican revolution. Since the law is a source of moral goodness, it follows that the good resides primarily in the will. To act is not only to be consistent with duty (law), but above all, to act *by* duty (morality). Virtue is deserving of happiness, and together they constitute the supreme good of man. By no means must the end justify the means, but rather, happiness (the end) and will (the means) must go hand in hand.

Postulates of Practical Reason

It is not the search for happiness that makes men virtuous, nor is virtue a sufficient guarantee of happiness. This approach leads to claims that the moral postulates:

  • The immortality of the soul: As a guarantee of indefinite progress in virtue.
  • God’s existence: As a guarantee that virtue and happiness will finally coincide.
  • Freedom: Considered both as a postulate and as something derived from the existence of moral law.