When practical reason thinks itself into a world of understanding, it does not thereby transcend its own limits, as it would if it tried to enter it by intuition or sensation. The former is only a negative thought in respect of the world of sense, which does not give any laws to reason in determining the will and is positive only in this single point that this freedom as a negative characteristic is at the same time conjoined with a (positive) faculty and even with a causality of reason, which we designate a will, namely a faculty of so acting that the principle of the actions shall conform to the essential character of a rational motive, i.e., the condition that the maxim have universal validity as a law. But were it to borrow an object of will, that is, a motive, from the world of understanding, then it would overstep its bounds and pretend to be acquainted with something of which it knows nothing. The conception of a world of the understanding is then only a point of view which reason finds itself compelled to take outside the appearances in order to conceive itself as practical, which would not be possible if the influences of the sensibility had a determining power on man, but which is necessary unless he is to be denied the consciousness of himself as an intelligence and, consequently, as a rational cause, energizing by reason, that is, operating freely. This thought certainly involves the idea of an order and a system of laws different from that of the mechanism of nature which belongs to the sensible world; and it makes the conception of an intelligible world necessary (that is to say, the whole system of rational beings as things in themselves). But it does not in the least authorize us to think of it further than as to its formal condition only, that is, the universality of the maxims of the will as laws, and consequently the autonomy of the latter, which alone is consistent with its freedom; whereas, on the contrary, all laws that refer to a definite object give heteronomy, which only belongs to laws of nature and can only apply to the sensible world. But reason would overstep all its bounds if it undertook to explain how pure reason can be practical, which would be exactly the same problem as to explain how freedom is possible.