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)