The most famous of all universality tests is Kant’s Categorical Imperative. He formulates the test in a number of ways and claims that they are equivalent. I shall not examine this claim, but shall concentrate on the main formulation of the test which explicitly uses the notion of universality, but does not explicitly use any of the other central Kantian notions, such as autonomy, treating others as ends, acting as a legislator in the kingdom of ends, and so on. This formulation of the test is the Formula of Universal Law. It runs:

1. I. Act only on that maxim through which you can at the same time will that it should become a universal law.

A second and subsidiary formulation, which also depends on the notion of universality, is the Formula of the Law of Nature. It runs:

1. Ia. Act as if the maxim of your action were to become through your will a universal law of nature.

Some justification is needed for a decision to concentrate on Formulas i and ia. Many would claim that these are the least interesting, the most sterile and ‘formalistic’ versions of the Categorical Imperative. To concentrate on these, it is said, is to miss the humanistic core of Kant’s ethics, and attention should rather be paid to the notions of autonomy, treating others as ends, acting as a legislator, and so on. I have two reasons for disregarding such advice. First, I do not believe that Formulas i and ia are sterile. Chapter 5 will show the extent to which they can be action-guiding. Second, the formalism, such as it is, of these two formulas is one of their merits. It is easier to work out the implications of (relatively) formal requirements than of other sorts of requirements. If Formulas i and ia lead nowhere, it should at least be possible to determine this quite precisely. These formulas are, Kant says, the ‘strictest’ versions of the Categorical Imperative, though others may be more useful for securing acceptance of the moral law.