I first came to think about acting on principle on the rebound from a brief and strong enthusiasm for utilitarianism. I was impressed with the scope, fertility and precision of that ethical theory; then distressed by its strong and implausible premises. The very precision which had beguiled me now seemed spurious and hence dangerous. But I remained sure that a moral theory which was not fruitful, which could not guide action, was pointless. Without much hope I began looking at some Kantian ethical theories, such as those of Baier and Hare, and parts of Singer. These theories construe the supreme principle of morality as enjoining a test of principles by appeal to one or another concept of universality. Some of these theories met my demand for plausibility, but none seemed able to guide action. Even though they might show which principles were moral principles, they did not show how to determine which acts a person ought or might do. Most of these theories lacked any account of the connection between principles and acts. They could not determine which of the many principles that apply to a given act it was relevant to assess; and without a solution to the problem of relevance universality tests of principles are impotent.

I had little hope that Kant’s own ethical theory would satisfy my demands any better than recent and, as I supposed, clearer theories. Kant’s commentators and imitators, while sharing his enthusiasm for the lofty austerity of the Categorical Imperative, have mainly regarded it as a failure. Some have thought it too meagre a principle to guide action; others that it led straight to rigourism – to a senseless uniformity of action which disregards the subtle diversity of human circumstances. As I worked on Kant’s writings, I came to believe that neither of these charges can be made to stick. The Categorical Imperative can guide action and does not lead to rigourism.