The Rule Deontological moral theory proposed by the German philosopher Immanuel Kant (1724-1804) is most commonly known simply as Kantianism.

According to Kant, normative evaluation is a matter of pratical reason which articulates to all rational beings a single, absolute, objective moral principle called the Categorical Imperative. For Kant, to be truly moral, a principle must be good-in-itself, good for its own sake, regardless of the consequences. This is what distinguishes the Categorical Imperative from its cousin the Hypothetical Imperative (one which is good for the sake of that which it accomplishes). The CI is given three distinct formulations by Kant in his famous work, The Groundwork of the Metaphysics of Morals:

Any particular action, in order to be moral, must be motivated by the CI. That means that two identical actions may not have the same moral status. Suppose that two people, while in conversation with someone, tell the truth. Person 1 tells the truth because he believes that truth-telling is a universal moral obligation which everyone ought to obey. On the other hand, perons 2 tells the truth because she hopes to be rewarded for her honesty. According to Kant P1 is acting morally, while P2 is not; P1 has acted according to the CI while P2 has acted according to an HI.

Kant’s Deontological morality is often described as absolutist because the origin of moral obligation (the CI) lies in the universal structure of pure practical reason which, like mathematics (which is in the realm of pure reason), is not relative to culture, history, biology or any other limiting factor. Just as 2+2 always equals 4, pure practical reasoning always dictates the CI.