To argue that a thing is good because it is natural, or bad because it is unnatural, in these common senses of the term, is therefore certainly fallacious; and yet such arguments are very frequently used. But they do not commonly pretend to give a systematic theory of Ethics. Among attempts to systematise an appeal to nature, that which is now most prevalent is to be found in the application to ethical questions of the term Evolution—in the ethical doctrines which have been called Evolutionistic. These doctrines are those which maintain that the course of evolution, while it shews us the direction in which we are developing, thereby and for that reason shews us the direction in which we ought to develop. Writers, who maintain such a doctrine, are at present very numerous and very popular; and I propose to take as my example the writer, who is perhaps best known of them all—Mr Herbert Spencer. Mr Spencer’s doctrine, it must be owned, does not offer the clearest example of the naturalistic fallacy as used in support of Evolutionistic Ethics. A clearer example might be found in Guyau, a writer who has lately had considerable vogue in France, but who is not so well known as Spencer. Guyau might almost be called a disciple of Spencer; he is frankly evolutionistic, and frankly naturalistic; and I may mention that he does not seem to think that he differs from Spencer by reason of his naturalism. The point in which he has criticised Spencer concerns the question how far the ends of pleasure and of increased life coincide as motives and means to the attainment of the ideal: he does not seem to think that he differs from Spencer in the fundamental principle that the ideal is Quantity of life, measured in breadth as well as in length, or, as Guyau says, Expansion and intensity of life; nor in the naturalistic reason which he gives for this principle. And I am not sure that he does differ from Spencer in these points. Spencer does, as I shall shew, use the naturalistic fallacy in details; but with regard to his fundamental principles, the following doubts occur: Is he fundamentally a Hedonist? And, if so, is he a naturalistic Hedonist? In that case he would better have been treated in my next chapter. Does he hold that a tendency to increase quantity of life is merely a criterion of good conduct? Or does he hold that such increase of life is marked out by nature as an end at which we ought to aim? (§ 29 ¶ 1)

I think his language in various places would give colour to all these hypotheses; though some of them are mutually inconsistent. I will try to discuss the main points. (§ 29 ¶ 2)