The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 55

it proves that we do really recognise the validity of the categorical imperative and (with all respect for it) only allow ourselves a few exceptions, which we think unimportant and forced from us.

We have thus established at least this much, that if duty is a conception which is to have any import and real legislative authority for our actions, it can only be expressed in categorical and not at all in hypothetical imperatives. We have also, which is of great importance, exhibited clearly and definitely for every practical application the content of the categorical imperative, which must contain the principle of all duty if there is such a thing at all. We have not yet, however, advanced so far as to prove a priori that there actually is such an imperative, that there is a practical law which commands absolutely of