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.

For we can explain nothing but that which we can reduce to laws, the object of which can be given in some possible experience. But freedom is a mere idea, the objective reality of which can in no wise be shown according to laws of nature, and consequently not in any possible experience; and for this reason it can never be comprehended or understood, because we cannot support it by any sort of example or analogy. It holds good only as a necessary hypothesis of reason in a being that believes itself conscious of a will,