The name Moral Philosophy naturally suggests that what is meant is a department of philosophy which has something to do with morality. And we all understand roughly what is meant by morality. We are accustomed to the distinction between moral good and evil, on the one hand, and what is sometimes called physical good and evil on the other. We all make the distinction between a man's moral character, on the one hand, and his agreeableness or intellectual endowments, on the other. We feel that to accuse a man of immoral conduct is quite a different thing from accusing him merely of bad taste or bad manners, or from accusing him merely of stupidity or ignorance. And no less clearly we distinguish between the idea of being under a moral obligation to do a thing, and the idea of being merely under a legal obligation to do it. It is a commonplace that the sphere of morality is much wider than the sphere of law: that we are morally bound to do and avoid many things, which are not enjoined or forbidden by the laws of our country; and it is also sometimes held that, if a particular law is unjust or immoral, it may even be a moral duty to disobey it--that is to say that there may be a positive conflict between moral and legal obligation; and the mere fact that this is held, whether truly or falsely, shows, at all events, that the one idea is quite distinct from the other. The name Moral Philosophy, then, naturally suggests that it is a department of philosophy concerned with morality in this common sense. And it is, in fact, true that one large department of Moral Philosophy is so concerned. But it would be a mistake to think that the whole subject is _only_ concerned with morality. Another important department of it is, as I shall try to show, concerned with ideas which are _not_ moral ideas, in this ordinary sense, though, no doubt, they may have something to do with them. And of the two ideas which I propose to pick out for discussion, while one of them is a moral idea, the other belongs to that department of Moral Philosophy, which is not concerned solely with morality, and is not, I think, properly speaking, a moral idea at all. Let us begin with the one of the two, which is a moral idea. The particular moral idea which I propose to pick out for discussion is the one which I have called above the idea of moral obligation--the idea of being morally bound to act in a particular way on a particular occasion. But what is, so far as I can see, precisely the same idea is also called by several other names. To say that I am under a moral obligation to do a certain thing is, I think, clearly to say the same thing as what we commonly express by saying that I ought to do it, or that it is my duty to do it. That is to say, the idea of moral obligation is identical with the idea of the moral "ought" and with the idea of duty. And it also seems at first sight as if we might make yet another identification.