It is just the same as if I sought to find out how freedom itself is possible as the causality of a will. For then I quit the ground of philosophical explanation, and I have no other to go upon. I might indeed revel in the world of intelligences which still remains to me, but although I have an idea of it which is well founded, yet I have not the least knowledge of it, nor an I ever attain to such knowledge with all the efforts of my natural faculty of reason. It signifies only a something that remains over when I have eliminated everything belonging to the world of sense from the actuating principles of my will, serving merely to keep in bounds the principle of motives taken from the field of sensibility; fixing its limits and showing that it does not contain all in all within itself, but that there is more beyond it; but this something more I know no further. Of pure reason which frames this ideal, there remains after the abstraction of all matter, i.e., knowledge of objects, nothing but the form, namely, the practical law of the universality of the maxims, and in conformity with this conception of reason in reference to a pure world of understanding as a possible efficient cause, that is a cause determining the will. There must here be a total absence of springs; unless this idea of an intelligible world is itself the spring, or that in which reason primarily takes an interest; but to make this intelligible is precisely the problem that we cannot solve. Here now is the extreme limit of all moral inquiry, and it is of great importance to determine it even on this account, in order that reason may not on the one hand, to the prejudice of morals, seek about in the world of sense for the supreme motive and an interest comprehensible but empirical; and on the other hand, that it may not impotently flap its wings 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. Concluding Remark