Kant conceives his investigation as a work of foundational ethics—one that clears the ground for future research by explaining the core concepts and principles of moral theory, and showing that they are normative for rational agents.

Central to the work is the role of what Kant refers to as the categorical imperative, which states that one must act only according to maxims which one could will to become a universal law.

The book is famously difficult,[citation needed] and it is partly because of this that Kant later, in 1788, decided to publish the Critique of Practical Reason.

Kant opens the preface with an affirmation of the Ancient Greek idea of a threefold division of philosophy into logic, physics, and ethics.

[ii] The search for the supreme principle of morality—the antidote to confusion in the moral sphere—will occupy Kant for the first two chapters of the Groundwork.

Because Kant believes that any fact that is grounded in empirical knowledge must be contingent, he can only derive the necessity that the moral law requires from a priori reasoning.

Common sense distinguishes among: Kant thinks our actions only have moral worth and deserve esteem when they are motivated by duty.

Kant contrasts the shopkeeper with the case of a person who, faced with “adversity and hopeless grief”, and having entirely lost his will to live, yet obeys his duty to preserve his life.

Kant also notes that many individuals possess an inclination to do good; but however commendable such actions may be, they do not have moral worth when they are done out of pleasure.

If, however, a philanthropist had lost all capacity to feel pleasure in good works but still did pursue them out of duty, only then would we say they were morally worthy.

[vii][viii] One interpretation asserts that the missing proposition is that an act has moral worth only when its agent is motivated by respect for the law, as in the case of the man who preserves his life only from duty.

If the shopkeeper in the above example had made his choice contingent upon what would serve the interests of his business, then his act has no moral worth.

For example, if a person wants to qualify for nationals in ultimate frisbee, he will have to follow a law that tells him to practice his backhand pass, among other things.

At this point, Kant asks, "what kind of law can that be, the representation of which must determine the will, even without regard for the effect expected from it...?

Thus, Kant arrives at his well-known categorical imperative, the moral law referenced in the above discussion of duty.

Kant defines the categorical imperative as the following:[x]I ought never to act except in such a way that I could also will that my maxim should become a universal law.Later, at the beginning of Section Two, Kant admits that it is in fact impossible to give a single example of an action that could be certainly said to have been done from duty alone, or ever to know one's own mind well enough to be sure of one's own motives.

In other words, only rational beings have the capacity to recognize and consult laws and principles in order to guide their actions.

For example, if a person wants to qualify for nationals in ultimate frisbee, he will recognize and consult the rules that tell him how to achieve this goal.

Kant believes that this leaves us with one remaining alternative, namely that the categorical imperative must be based on the notion of a law itself.

From this observation, Kant derives the categorical imperative, which requires that moral agents act only in a way that the principle of their will could become a universal law.

If an attempt to universalize a maxim results in a contradiction in conception, it violates what Kant calls a perfect duty.

However, in a later work (The Metaphysics of Morals), Kant suggests that imperfect duties only allow for flexibility in how one chooses to fulfill them.

The Formula of Autonomy combines the objectivity of the former with the subjectivity of the latter and suggests that the agent ask what he or she would accept as a universal law.

Because alien forces could only determine our actions contingently, Kant believes that autonomy is the only basis for a non-contingent moral law.

Finally, Kant remarks that whilst he would like to be able to explain how morality ends up motivating us, his theory is unable to do so.

Whilst humans experience the world as having three spatial dimensions and as being extended in time, we cannot say anything about how reality ultimately is, from a god's-eye perspective.

[citation needed] His criticism is an attempt to prove, among other things, that actions are not moral when they are performed solely from duty.

While he publicly called himself a Kantian, and made clear and bold criticisms of Hegelian philosophy, he was quick and unrelenting in his analysis of the inconsistencies throughout Kant's long body of work.

[2] He takes it to be a peculiar expression of "slavish" egalitarianism, de facto always already prioritizing the sick, the weakly over the healthy and strong – those capable of valid self-legislation to begin with –, thereby undermining the very possibility of human greatness at its root.

But others have stressed many deeper similarities that adherents to a framework of unqualified liberalism, prone to condemning Nietzsche from the canon, have overlooked.