Kant's Moral Law and the Categorical Imperative

The necessity of an a priori moral law

None of Kant's ethical claims can be established prior to the moral law. Kant explains why: because all empirical ethics depend on experience. No ethics with specific content can be universal and necessary. So how must ethics be to have a universal and necessary moral law? You can have the moral force of law only in an a priori principle; it must be purely formal because it cannot consist in particular content.

Duty and moral action

A moral action is an action from duty. Kant is quite clear: an action is moral only when the motive of the will is duty for duty's sake. Every rational being has at some time felt the supremacy of the force of duty.

The categorical imperative and its character

The moral law, for Kant, is a fact of practical reason. It is a principle of action (practice) that is universal, strict (no exceptions), and absolutely necessary. It is given a priori in reason; it must be an absolutely unconditional imperative, not depending on anything else. This is called the categorical imperative.

The form of the categorical imperative cannot order any specific action directly; it must be expressed as a formal principle rather than by mandating particular ends.

Key formulations of the categorical imperative

• Formula of Universal Law: "Act only according to that maxim whereby you can at the same time will that it should become a universal law."
• Formula of Humanity: "Act so that you treat humanity always as an end and never merely as a means."

Practical postulates: Freedom, immortality, and God

Kant introduces practical postulates that must be presupposed to make sense of moral cognition and moral striving.

I. First postulate: Freedom

Freedom. No one can act from duty unless they are free. Freedom is the condition that makes the moral law possible. We have no empirical experience of freedom; we cannot study freedom scientifically, yet without the presupposition of freedom the moral law given to us would be impossible.

II. Second postulate: The immortality of the soul

The immortality of the soul. Kant posits the immortality of the soul as a condition that makes possible the ultimate ends of practical reason.

III. Third postulate: The existence of God

The existence of God. God functions as the guarantee that the laws of nature (what is) and the laws of the will (what ought to be) can ultimately converge under a single legislator: God.

Postulates and the ultimate ends of the will

These postulates are conditions that make possible the ultimate ends of the will. For example, the union of happiness with virtue—which are often not yet joined—appears as an end that reason requires. Our will contains this purpose as something to be achieved. Kant held that failure to achieve this union in this life can be remedied in a future existence after death.