The fourth chapter deals with "Metaphysical Ethics." "The main object of this chapter has been to show that Metaphysics, understood as the investigation of a supposed supersensible reality, can have no logical bearing whatever upon the answer to the fundamental ethical question 'What is good in itself?' That this is so, follows at once from the conclusion of Chapter I., that ’good' denotes an ultimate, unanalysable predicate; but this truth has been so systematically ignored, that it seemed worth while to discuss and distinguish in detail the principal relations, which do hold, or have been supposed to hold, between Metaphysics and Ethics. With this view I pointed out:—(1) That Metaphysics may have a bearing on practical Ethics—on the question 'What ought we to do?'—so far as it may be able to tell us what the future effects of our action will be: what it can not tell us is whether those effects are good or bad in themselves. One particular type of metaphysical doctrine, which is very frequently held, undoubtedly has such a bearing on practical Ethics: for, if it is true that the sole reality is an eternal, immutable Absolute, then it follows that no action of ours can have any real effect, and hence no practical proposition can be true. The same conclusion follows from the ethical proposition, commonly combined with this metaphysical one—namely, that this eternal Reality is also the sole good. (2) That metaphysical writers, as where they fail to notice the contradiction just noticed between any practical proposition and the assertion that an eternal reality is the sole good, seem frequently to confuse the proposition that one particular existing thing is good, with the proposition that the existence of that kind of thing would be good, wherever it might occur. To the proof of the former proposition Metaphysics might be relevant, by shewing that the thing existed; to the proof of the latter it is wholly irrelevant: it can only serve the psychological function of suggesting things which may be valuable—a function which would still be better performed by pure fiction.
"But the most important source of the supposition that metaphysics is relevant to Ethics, seems to be the assumption that 'good' must denote some real property of things—an assumption which is mainly due to two erroneous doctrines, the first logical, the second epistemological. Hence (3) I discussed the logical doctrine that all propositions assert a relation between existents; and pointed out that the assimilation of ethical propositions either to natural laws or to commands are instances of this logical fallacy. And finally (4) I discussed the epistemological doctrine that to be good is equivalent to being willed or felt in some particular way; a doctrine which derives support from
