The fallacy of supposing moral law to be analogous to natural law in respect of asserting that some action is one which is always necessarily done is contained in one of the most famous doctrines of Kant. Kant identifies what ought to be with the law according to which a Free or Pure Will must act—with the only kind of action which is possible for it. And by this identification he does not mean merely to assert that Free Will is also under the necessity of doing what it ought; he means that what it ought to do means nothing but its own law—the law according to which it must act. It differs from the human will just in that, what we ought to do, is what it necessarily does. It is autonomous; and by this is meant (among other things) that there is no separate standard by which it can be judged: that the question Is the law by which this Will acts a good one? is, in its case, meaningless. It follows that what is necessarily willed by this Pure Will is good, not because that Will is good, nor for any other reason; but merely because it is what is necessarily willed by a Pure Will. (§ 75 ¶ 1)

Kant's assertion of the Autonomy of the Practical Reason thus has the very opposite effect to that which he desired; it makes his Ethics ultimately and hopelessly heteronomous. His Moral Law is independent of Metaphysics only in the sense that according to him we can knnow it independently; he holds that we can only infer that there is Freedom, from the fact that the Moral Law is true. And so far as he keeps strictly to this view, he does avoid the error, into which most metaphysical writers fall, of allowing his opinions as to what is real to influence his judgments of what is good. But he fails to see that on his view the Moral Law is dependent upon Freedom in a far more important sense than that in which Freedom depends on the Moral Law. He admits that Freedom is the ratio essendi of the Moral Law, whereas the latter is only the ratio cognoscendi of Freedom. And this means that, unless Reality be such as he says, no assertion that This is good can possibly be true: it can indeed have no meaning. He has, therefore, furnished his opponents with a conclusive method of attacking the validity of the Moral Law. If they can only shew by some other means (which he denies to be possible but leaves theoretically open) that the nature of Reality is not such as he says, he cannot deny that they will have proved his ethical principle to be false. If what This ought be done means This is willed by a Free Will, then, if it can be shewn that there is no Free Will which wills anything, it will follow that nothing ought to be done. (§ 75 ¶ 2)