A pair contrasted in Kantian ethics. A hypothetical imperative embeds a command which is in place only given some antecedent desire or project: ‘If you want to look wise, stay quiet.’ The injunction to stay quiet only applies to those with the antecedent desire or inclination; if one has no desire to look wise the injunction or advice lapses. A categorical imperative cannot be so avoided: it is a requirement that binds anybody, regardless of their inclinations. It could be represented as, for example: ‘Tell the truth! (regardless of whether you want to or not).’ The distinction is not always signalled by presence or absence of the conditional or hypothetical form: ‘if you crave drink, don’t become a bartender’ may be regarded an absolute injunction applying to anyone, although only activated in the case of those with the stated desire.

In Grundlegung zur Metaphysik der Sitten (1785), Kant discussed five forms of the categorical imperative:

1. (i) the formula of universal law: ‘act only on that maxim through which you can at the same time will that it should become universal law’;

2. (ii) the formula of the law of nature: ‘act as if the maxim of your action were to become through your will a universal law of nature’;

3. (iii) the formula of the end-in-itself: ‘act in such a way that you always treat humanity, whether in your own person or in the person of any other, never simply as a means, but always at the same time as an end’;

4. (iv) the formula of autonomy, or considering ‘the will of every rational being as a will which makes universal law’; and

5. (v) the formula of the Kingdom of Ends, which provides a model for the systematic union of different rational beings under common laws.

A central object of the study of Kant’s ethics is to understand these expressions of the inescapable, binding requirement of the categorical imperative, and to understand whether they are equivalent at some deep level. Kant’s own applications of the notion are not always convincing (see, for example, sex). One cause of confusion in relating Kant’s ethics to theories such as expressivism is that it is easy, but mistaken, to suppose that the categorical nature of the imperative implies that it cannot be the expression of a sentiment, but must instead derive from something ‘unconditioned’ or ‘necessary’ such as the voice of reason.