That such a reduction of all propositions to the type of those which assert either that something exists or that something which exists has a certain attribute (which means, that both exist in a certain relation to one another), is erroneous, may easily be seen by reference to the particular class of ethical propositions. For whatever we may have proved to exist, and whatever two existents we may have proved to be necessarily connected with one another, it still remains a distinct and different question whether what thus exists is good; whether either or both of the two existents is so; and whether it is good that they should exist together. To assert the one is plainly and obviously not the same thing as to assert the other. We understand what we mean by asking: Is this, which exists, or necessarily exists, after all, good? and we perceive that we are asking a question which has not been answered. In the face of this direct perception that the two questions are distinct, no proof that they must be identical can have the slightest value. That the proposition This is good is thus distinct from every other proposition was proved in Chapter I; and I may now illustrate this fact by pointing out how it is distinguished from two particular propositions with which it has commonly been identified. That so and so ought to be done is commonly called a moral law; and this phrase naturally suggests that the proposition is in some way analogous either to a natural law, or to a law in the legal sense, or to both. All three are, in fact, really analogous in one respect, and in one respect only: that they include a proposition which is universal. A moral law asserts This is good in all cases; a natural law asserts This happens in all cases; and a law, in the legal sense, It is commanded that this be done, or left undone, in all cases. But since it is very natural to suppose that the analogy extends further, and that the assertion This is good in all cases is equivalent to the assertion This happens in all cases or to the assertion It is commanded htat this be done in all cases, it may be useful briefly to point out that they are not equivalent. (§ 74 ¶ 1)

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)

And Kant also commits the fallacy of supposing that This ought to be means This is commanded. He conceives the Moral Law to be an Imperative. And this is a very common mistake. This ought to be, it is assumed, must mean This is commanded; nothing, therefore, would be good unless it were commanded; and since commands in this world are liable to be erroneous, what ought to be in its ultimate sense means what is commanded by some real supersensible authority. With regard to this authority it is, then, no longer possible to ask Is it righteous? Its commands cannot fail to be right, because to be right means to be what it commands. Here, therefore, law, in the moral sense, is supposed to be analogous to law, in the legal sense, rather than, as in the last instance, to law in the natural sense. It is supposed that moral obligation is analogous to legal obligation, with this difference only that whereas the source of legal obligation is earthly, that of moral obligation is heavenly. Yet it is obvious that if by a source of obligation is meant only a power which binds you or compels you to do a thing, it is not because it does do this that you ought to obey it. It is only if it be itself so good, that it commands and enforces only what is good, that it can be a source of moral obligation. And in that case what it commands and enforces would be good, whether commanded and enforced or not. Just that which makes an obligation legal, namely the fact that it is commanded by a certain kind of authority, is entirely irrelevant to moral obligation. However an authority be defined, its commands will be morally binding only if they are—morally binding; only if they tell us what ought to be or what is a means to that which ought to be. (§ 76 ¶ 1)