The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 49

without being able to move in the (for it) empty space of transcendent concepts which we call the intelligible world, and so lose itself amidst chimeras.

For the rest, the idea of a pure world of understanding as a system of all intelligences, and to which we ourselves as rational beings belong (although we are likewise on the other side members of the sensible world), this remains always a useful and legitimate idea for the purposes of rational belief, although all knowledge stops at its threshold, useful, namely, to produce in us a lively interest in the moral law by means of the noble ideal of a universal kingdom of ends in themselves (rational beings), to which we can belong as members then only when we carefully conduct ourselves according to the maxims of freedom as if they were laws of nature.