The study of Ethics is perhaps most commonly conceived as being concerned with the questions What sort of actions ought men to perform? and What sort of actions ought men to avoid? It is conceived, that is to say, as dealing with human conduct, and as deciding what is virtuous and what is vicious among the kinds of conduct between which, in practice, people are called upon to choose. Owing to this view of the province of ethics, it is sometimes regarded as the practical study, to which all others may be opposed as theoretical; the good and the true are sometimes spoken of as independent kingdoms, the former belonging to ethics, while the latter belongs to the sciences. (§ 1 ¶ 1)

This view, however, is doubly defective. In the first place, it overlooks the fact that the object of ethics, by its own account, is to discover true propositions about virtuous and vicious conduct, and that these are just as much a part of truth as true propositions about oxygen or the multiplication table. The aim is, not practice, but propositions about practice; and propositions about practice are not themselves practical, any more than propositions about gases are gaseous. One might as well maintain that botany is vegetable or zoology animal. Thus the study of ethics is not something outside science and co-ordinate with it: it is merely one among sciences. (§ 1 ¶ 2)