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. The moral idea, then, which I propose to discuss, is the idea of duty or moral obligation, or, what comes to the same thing, the idea of what is wrong--morally wrong. Everybody would agree that this idea--or, to speak more accurately, one or both of these two ideas--is among the most fundamental of our moral ideas, whether or not they would admit that all others, for example the ideas of moral goodness, involve a reference to this one in their definition, or would hold that we have some others which are independent of it, and equally fundamental with it.