The Groundwork for the Metaphysics of Morals was written in 1785, four years after Kant had written his magnum opus, The Critique of Pure Reason. In the Groundwork, Kant aims to extend the insights of the Critique. Where the Critique inquired into the human mind and asked what kind of knowledge we can legitimately claim, the Groundwork pushes into the realm of moral philosophy—asking what sorts of duties and obligations we have. Just as the Critique was supposed to clear away the claims of previous philosophies in order to assess what we can legitimately know and how we can know it, the purpose of the Groundwork was to clarify the core concepts of morality and to demonstrate what relationship morality had to human beings. The Groundwork would serve as the basis for his later, expanded work, the Critique of Practical Reason, which Kant published in 1788.
The Groundwork is usually understood as a response to the moral theories of the Scottish Enlightenment, particularly those of David Hume, whose skepticism Kant engaged in the critique, as well as the economist Adam Smith. Since both philosophers believed that all knowledge came from sensory experience, they believed that the same was true of moral judgments. According to the Scottish thinkers, whose ideas are known as the theory of moral sentiments, we consider good that which it gives us pleasure to do; this pleasure comes from the approval that we get for doing it. By the same token, we do good in order to avoid the bad feelings we would get from doing bad. For example, if we see someone suffering, we would feel bad if we did nothing, and the desire to avoid this bad feeling motivates us to do "good." Therefore, Hume and Smith say, morality does not have an objective basis. Kant argued against this: he wanted to prove that objective, universal moral judgments were possible, and that they were not rooted in feeling, but rather, in the form of the judgments themselves, which, Kant argued, was universal and unchanging.
For this reason, the Groundwork is also typically contrasted against the utilitarian school of philosophy, particularly Jeremy Benthem and John Stuart Mill, which came after Kant. The utilitarians argued that the goodness consisted in doing the greatest good for the greatest number. Kant’s universalist—or, as it is sometimes called, deontological—view of morality rejected the utilitarian view for the same reason that he rejected the theory of moral sentiments. Kant argues that to do good in hope of a good result is, in effect, to act in hopes of a reward, and therefore neither free nor truly moral.
The most famous idea in the Groundwork is the categorical imperative—the argument that only those actions are truly moral that can be the basis for a possible law. The categorical imperative is, in essence, a philosophical justification for the "golden rule": you should only commit those actions that you believe are universally justified. The concept has received enormous amounts of philosophical attention, from the time that it was published until today. Kant’s followers believed that he had successfully laid the basis for a universal morality, even if he hadn’t successfully shown that it was necessary to act morally. Famous readers like the poet Friedrich Schiller, as well as the philosophers G.W.F. Hegel and Friedrich Nietzsche, argued that the categorical imperative was cold, and based on a faulty universalism that denied individual experience.
Other aspects of the text have simply become part of our moral culture. The notion that treating others with dignity is inseparable from being aware of our dignity as human beings, as well as the idea that human dignity is inviolable, has been coded into numerous constitutions. Kant’s belief in a common humanity echoes the Enlightenment ideals of the eighteenth and nineteenth centuries. And, perhaps more subtly, echoes of the implication that one morality can be applicable not just for all individuals, but for all parts of an individual’s life, making no distinction for class, gender, race, or social position,, can be found in contemporary debates about politics in media or sexual harassment in the workplace.