Those theories of Ethics, then, are naturalistic which declare the sole good to consist in some one property of things, which exists in time; and which do so because they suppose that good itself can be defined by reference to such a property. And we may now proceed to consider such theories. (§ 27 ¶ 1)

And, first of all, one of the most famous of ethical maxims is that which recommends a life according to nature. That was the principle of the Stoic Ethics; but, since their Ethics has some claim to be called metaphysical, I shall not attempt to deal with it here. But the same phrase reappears in Rousseau; and it is not unfrequently maintained even now that what we ought to do is live naturally. Now let us examine this contention in its general form. It is obvious, in the first place, that we cannot say that everything natural is good, except perhaps in virtue of some metaphysical theory, such as I shall deal with later. If everything natural is equally good, then certainly Ethics, as it is ordinarily understood, disappears; for nothing is more certain, from an ethical point of view, than that some things are bad and others good; the object of Ethics is, indeed, in chief part, to give you general rules whereby you may avoid the one and secure the other. What, then, does natural mean, in this advice to live naturally, since it obviously cannot apply to everything that is natural? (§ 27 ¶ 2)

The phrase seems to point to a vague notion that there is some such thing as natural good; to a belief that Nature may be said to fix and decide what shall be good, just as she fixes and decides what shall exist. For instance, it may be supposed that health is susceptible of a natural definition, that Nature has fixed what health shall be: and health, it may be said, is obviously good; hence in this case Nature has decided the matter; we have only to go to her and ask her what health is, and we shall know what is good: we shall have based an ethics upon science. But what is this natural definition of health? I can only conceive that health should be defined in natural terms as the normal state of an organism; for undoubtedly disease is also a natural product. To say that health is what is preserved by evolution, and what itself tends to preserve, in the struggle for existence, the organism which possesses it, comes to the same thing: for the point of evolution is that it pretends to give a causal explanation of why some forms of life are normal and others are abnormal; it explains the origin of species. When therefore we are told that health is natural, we may presume that what is meant is that it is normal; and that when we are told to pursue health as a natural end, what is implied is that the normal must be good. But is it so obvious that the natural must be good? Is it really obvious that health, for instance, is good? Was the excellence of Socrates or of Shakespeare normal? Was it not rather abnormal, extraordinary? It is, I think, obvious in the first place, that not all that is good is normal; that, on the contrary, the abnormal is often better than the normal: peculiar excellence, as well as peculiar viciousness, must obviously be not normal but abnormal. Yet it may be said that nevertheless the normal is good; and I myself am not prepared to dispute that health is good. What I contend is that this must not be taken to be obvious; that it must be regarded as an open question. To declare it to be obvious is to suggest the naturalistic fallacy: just as in some recent books, a proof that genius is diseased, abnormal, has been used to suggest that genius ought not to be encouraged. Such reasoning is fallacious, and dangerously fallacious. The fact is that in the very words health and disease we do commonly include the notion that the one is good and the other bad. But, when a so-called scientific definition of them is attempted, a definition in natural terms, the only one possible is that by way of normal and abnormal. Now, it is easy to prove that some things commonly thought excellent are abnormal; and it follows that they are diseased. But it does not follow, except by virtue of the naturalistic fallacy, that those things, commonly thought good, are therefore bad. All that has really been shewn is that in some cases there is a conflict between the common judgment that genius is good, and the common judgment that health is good. It is not sufficiently recognised that the latter judgment has not a whit more warrant for its truth than the former; that both are perfectly open questions. It may be true, indeed, that by healthy we do commonly imply good; but that only shews that when we so use the word, we do not mean the same thing by it as the thing which is meant in medical science. That health, when the word is used to denote something good, is good, goes no way at all to shew that health, when the word is used to denote something normal, is also good. We might as well say that, because bull denotes an Irish joke and also a certain animal, the joke and the animal must be the same thing. We must not, therefore, be frightened by the assertion that a thing is natural into the admission that it is good; good does not, by definition, mean anything that is natural; and it is therefore always an open question whether anything that is natural is good. (§ 27 ¶ 3)