that there is a great difference between them: that whereas rules of duty do directly assert of the idea of duty, in the sense in which to say that something is your duty implies that you _can_ do it, that certain things are duties, the "ideal" rules do _not_ assert this, but something different. Yet the "ideal" rules certainly do, in a sense, assert a "moral obligation." And hence we have to recognise that the phrase "moral obligation" is not merely a name for one idea only, but for two very different ideas; and the same will, of course, be true of the corresponding phrase "morally wrong." When, therefore, I say that the idea of "moral obligation" is one of the fundamental ideas with which Moral Philosophy is concerned, I think we must admit that this one name really stands for two different ideas. But it does not matter for my purpose which of the two you take. Each of them is undoubtedly a moral idea, and whatever answer be given to the question we are going to raise about the one, will also certainly apply to the other. But it is now time to turn to the other idea, with which I said that Moral Philosophy has been largely concerned, though it is not, strictly speaking, a moral idea, at all. And I think, perhaps, a good way of bringing out what this idea is, is to refer to the Ethics of Aristotle. Everybody would admit that the fundamental idea, with which Aristotle's Ethics is concerned, is an idea which it is the business of Moral Philosophy to discuss; and yet I think it is quite plain that this idea is not a moral idea at all. Aristotle does not set out from' the idea of moral obligation or duty (indeed throughout his treatise he only mentions this idea quite incidentally); nor even from the idea of moral goodness or moral excellence, though he has a good deal more to say about that; but from the idea of what he calls "the human good," or "good for man." He starts by raising the question what the good for man _is,_ and his whole book is arranged in the form of giving a detailed answer to that question. And I think we can gather pretty well what the idea is, which he calls by this name, by considering what he says about it. There are two points, in particular, which he insists upon from the outset: first, that nothing can be good, in the sense he means, unless it is something which is worth having for its own sake, and not merely for the sake of something else; it must be good _in itself_; it must not, like wealth (to use one example which he gives) be worth having merely for the sake of what you can do with it; it must be a thing which is worth having even if nothing further comes of it. And secondly (what partly covers the former, but also, I think, says something more) it must, he says, be something that is "self-sufficient": something which, even if you had nothing else would make your life worth having. And further light is thrown upon his meaning when he comes to tell you