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.
The assertion that I ought to do a certain thing seems as if it meant much the same as the assertion that it would be wrong of me _not_ to do the thing in question: at all events it is quite clear that, whenever it is my duty to do anything, it would be wrong of me not to do it, and that whenever it would be wrong of me to do anything, then it is my duty to refrain from doing it. In the case of these two ideas, the idea of what is wrong, and the idea of what is my duty or what I ought to do, different views may be taken as to whether the one is more fundamental than the other, or whether both are equally so; and on the question: _If_ one of the two is more fundamental than the other, which of the two is so? Thus some people would say, that the idea of "wrong" is the more fundamental, and that the idea of "duty" is to be defined in terms of it: that, in fact, the statement "It is my duty to keep that promise" merely means "It would be wrong of me not to keep it"; and the statement "It is my duty not to tell a lie" merely means "It would be wrong of me to tell one." Others again would apparently say just the opposite: that duty is the more fundamental notion, and "wrong" is to be defined in terms of it. While others perhaps would hold that neither is more fundamental than the other; that both are equally fundamental, and that the statement "it would be wrong to do so and so" is only equivalent to, not identical in meaning with, "I ought not to do it." But whichever of these three views be the true one, there is, I think, no doubt whatever about the equivalence notion of the two ideas; and no doubt, therefore, that whatever answer be given to the question I am going to raise about the one, the same answer must be given to the corresponding question about the other.