The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 51

condition, the satisfaction of reason is only further and further postponed. Hence it unceasingly seeks the unconditionally necessary and finds itself forced to assume it, although without any means of making it comprehensible to itself, happy enough if only it can discover a conception which agrees with this assumption. It is therefore no fault in our deduction of the supreme principle of morality, but an objection that should be made to human reason in general, that it cannot enable us to conceive the absolute necessity of an unconditional practical law (such as the categorical imperative must be).

It cannot be blamed for refusing to explain this necessity by a condition, that is to say, by means of some interest assumed as a basis, since the law would then cease to be a supreme law of reason.