Kant: The Categorical Imperative and the Philosophy of Pure Practical Reason
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Kant: The Categorical Imperative and the Philosophy of Pure Practical Reason

A brief summary of Immanuel Kant's philosophy of pure practical reason as it is discussed in his book, Groundwork on the Metaphysics of Morals. Included are explanations of his three formulations, the hypothetical and categorical imperatives, the Formula for Universality, the Formula for Humanity and the Kingdom of Ends.

Immanuel Kant is widely considered one of the most important moral philosophers to have ever lived. He is one of the most influential philosophers of the modern era, and his body of work has had profound influence on the philosophers who followed him. In Groundwork on the Metaphysics of Morals, Kant lays out in three sections what is considered to be his crowning achievement, his philosophy of pure practical reason. Kant seeks to ground morality in reason, denouncing empiricists like Hume in favor of a system governing how we ought to conceive of and attain moral goodness through reason. He begins by exploring the foundations of moral reasoning, the inherent value of a good will, and how we are to go about making moral decisions. He advocates what he calls the categorical imperative, supported by the formula of universality. He then continues on to explore why we ought to reason in this way. The system’s foundation rests on the fundamental value of human beings as rational agents, and works to establish what Kant calls a “kingdom of ends;” a scenario in which all rational agents cooperate together to create a morally-ordered world. He concludes by claiming that the categorical imperative requires one always to act so that she regards herself as the legislator of her universal maxims in the Kingdom of Ends.

Kant’s foray into the functioning of moral reasoning begins with an establishment of the fact that it is indeed reasoning, and not observation. The empiricists would have it that all knowledge can be grounded in experience, but Kant is decidedly against this. To Kant, the main concern of morality is to determine how we ought to act, not to observe how we do. Therefore experience has no application to morality; all moral reasoning is exactly that, reasoning. Kant defines morality by its synthetically a priori nature. He declares moral judgments as synthetic because they endeavor to uncover some knowledge about our conceptions that is not predicated by the subject itself. If moral issues were analytic rather than synthetic, we would be able to solve all moral dilemmas simply by clarifying our concepts.

As a result of the normative nature of morality, Kant explains that there must be a governing principle for what is moral and what is not. This moral measuring stick is what he calls the categorical imperative. This imperative takes the aforementioned form of a synthetic, a priori principle for morality. Throughout the three sections, Kant provides three formulations for this imperative. The first formulation states that a good will functioning in accordance with duty requires one to always act in a way that her action can be made into a maxim, and that her maxim be capable of being universalized for all rational beings. There are several different claims at work in this formulation.

The first is that all rational agents place value on a good will. Kant elucidates this by pointing out that when we admire the actions of a person, we admire them because that person’s motive was itself admirable. For example, if a man were to run into a fire to save a child, but did so only because he thought he would receive a reward, we would not admire him. In fact, what we do admire is that the act performed was in itself good. This, to Kant, means that the admirable person’s motive proceeds from a sense of duty. The good will is one that acts according to what is inherently good, regardless of the consequences or other motivations, such as a reward, self-gratification, emotion, etc. If the act was performed with a view of achieving some other end, the imperative then becomes what Kant calls hypothetical. A hypothetical imperative is one in which the person acts according to a principle that she wills to achieve some end, and also therefore wills the means to that end. Since the categorical imperative is the one on which morality is based, Kant concurs that the person acting from duty and a having a good will must act according to that imperative, not the hypothetical.

The larger claim is mainly that one must act so that her actions can be formulated into a universal maxim. The rational agent must assume her action to be a maxim, and then envision a world in which everyone always acted according to that maxim. If the world in which this maxim is universalized turns out to be one that is in conflict with reason, the maxim and therefore the action should not and cannot be universalized. Under this formula of universality, in order to determine whether an action is morally permissible, the agent must conceive of it as an unconditional law. Essentially, Kant asks of the rational agent that she consider what the world would be like if everyone acted according to the maxim she proposes. If universalizing the act would cause it to lose its application altogether (as in the case of proposing everyone lie to get money when they need it, the consequence being that the lie would become ineffective because no one trusts a promise anymore) it fails the test of universality. We have a duty to act in ways that our maxims can be universalized; these are perfect duties. They are not optional, and are essential for a good will. We also have an obligation to imperfect duties. These are maxims that can be abstractly universalized (one should help others), but cannot be fulfilled all the time. Kant says we have selective power over when we perform these duties, but that we perform is not negotiable.

The second formulation Kant proposes serves to address the issue of why a rational agent should be motivated at all by duty. For this, he invokes the Formula of Humanity, which mandates that one always treat others as ends in themselves, never as a mere means to an end. The implication here is that we see human beings as intrinsically valuable and worthy of respect. If one were to act in a way that transgressed the autonomy and dignity of another, she would be in violation of the Formula of Humanity. An example of this can be seen when one examines the act of lying. If someone chooses to lie to another person, she is in effect excluding that person from participation in her reasoning. Rather than treating that person as a means in himself, she chooses to treat him as an object she can manipulate to her own gain. Upon examining this formulation, one may think that Kant is envisioning a kind of perfect moral world where all rational agents respect one another and mutually strive toward moral order. This is actually not far from true; Kant calls it the Kingdom of Ends. Kant describes this kingdom as a world in which all agents respect the autonomy inherent to all rational beings as a result of their ability to will universal maxims. Because of this mutual respect, those in the kingdom only will ends that do not violate the agency of other people, thus creating moral order.

The third and final formulation of Kant’s categorical imperative deals with the legislative duties of rational agents. He emphasizes that all agents be wedded to the idea that they function as legislators of their maxims in the Kingdom of Ends. This argument derives from Kant’s elucidation of the noumenal and phenomenal worlds, the former being the essential world, or the world of understanding and the latter being the sensible world. Kant says that we consider ourselves members of both. Our membership to the phenomenal gives us our knowledge world and of causal laws, but our status as rational beings capable of conferring value to our actions necessarily leads us to associate ourselves with the noumenal world of understanding. It is through this association that we define ourselves as legislators, and therefore autonomous, so intrinsically valuable as ends in ourselves. Our attachment to the world of understanding and our standing as rational, autonomous agents is founded upon our connection to the phenomenal world of sense. In order to belong to both these worlds and the Kingdom of Ends, we govern ourselves with universal moral maxims. Thus Kant establishes that we do indeed have obligations, and that they are intrinsic to our nature.

In summary, Kant’s three formulations of the categorical imperative build upon one another and arrive at what he believes is a comprehensive, normative system of morality. We first must act only in ways where our actions can be made into law, and that law can be universalized. Then we must always remember to treat other rational beings as inherently valuable and never use them as mere means to our ends. Finally we must keep in perspective that when we act, we must consider ourselves legislators in the Kingdom of Ends, and thus obligated to keep moral order.


Kant, Immanuel. Groundwork of the Metaphysics of Morals. 16th Edition. Cambridge University Press. Cambridge, United Kingdom. 2010. Section 1-3. 

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