Full Idea
With the categorical imperative or law of morality we have a very serious difficulty, because it is a synthetic a priori practical proposition.
Gist of Idea
The categorical imperative is a practical synthetic a priori proposition
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 420.50)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.83