Its 77 Academy pages make the Groundwork of the Metaphysics of Morals a short book, certainly by Kantian standards. Readers of the Critique of Pure Reason or the Metaphysics of Morals can easily get the impression – true or not – that they are looking at a ‘patchwork’ of previously existing material. The Groundwork is different. It was composed with great care. Moreover, Kant's technical language is absent from the first section, whereas the second employs resounding concepts like that of human beings as ‘ends in themselves’ or that of a ‘kingdom of ends’ that we are morally bound to create through our actions. These qualities explain its enduring popularity.

At the same time, the Groundwork claims to be as revolutionary in the field of ethics as the Critique of Pure Reason was in theoretical philosophy. Kant argues that all other ethical theories are fundamentally unsound because they fail to separate the rational and the natural elements of human volition. An unconditional moral command – a ‘categorical imperative’ – can only be grounded in pure reason. But this revolution concerns the level of ethical theory, not that of morality. Kant claims to re-establish what he claims are the insights of an uncorrupted common understanding of value and duty against the dangerous perversions peddled by his philosophical opponents. He emphasizes the capacity for self-determination or ‘autonomy’ that is located within individual human beings; and yet, the law that we impose upon ourselves is not arbitrary, it commands with unrelenting necessity.