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.
But there is a good deal of difficulty in getting clear as to what this idea of moral obligation itself is. Is there in fact only one idea which we call by this name? Or is it possible that on some occasions when we say that so and so is a duty, we mean something different by this expression from what we do on others? And that similarly when we say that so and so is morally wrong, we sometimes use this name "morally wrong" for one idea and sometimes for another; so that one and the same thing may be "morally wrong" in one sense of the word, and yet _not_ morally wrong in another? I think, in fact, there are two different senses in which we use these terms; and to point out the difference between them, will help to bring out clearly more the nature of each. And I think perhaps the difference can be brought out most clearly by considering the sort of moral rules with which we are all of us familiar.
Everybody knows that moral teachers are largely concerned in laying down moral rules, and in disputing the truth of rules which have been previously accepted. And moral rules seem to consist, to a very large extent, in assertions to the effect that it is always wrong to do certain actions or to refrain from doing certain others; or (what comes to the same thing) that it is always your duty to refrain from certain actions, and positively to do certain others. The Ten Commandments for example, are instances of moral rules; and most of them are examples of what are called negative rules--that is to say rules which assert merely that it is wrong to do certain positive actions, and therefore our duty to refrain from these actions; instead of rules which assert of certain positive actions, that it is our duty to do them and therefore wrong to refrain from doing them. The fifth commandment, which tells us to honour our father and mother, is apparently an exception; it seems to be a positive rule. It is not, like the others, expressed in the negative form "Thou shalt _not_ do so and so," and it is apparently really meant to assert that we ought to do certain positive actions, not merely that there are some positive action from which we ought to refrain. The difference between this one and the rest will thus serve as an example of the difference between positive and negative moral rules, a difference which is sometimes treated as if it were of great importance. And I do not wish to deny that there may be some important difference between seeing only that certain positive actions are wrong, and seeing also that, in certain cases, to refrain from doing certain actions is just as wrong as positively to do certain others. But this distinction between positive and negative rules is certainly of much less importance than another which is, I think, liable to be confused with it. So far as this distinction goes it is only a distinction between an assertion that it is wrong to do a positive action and an assertion that it is wrong to refrain from doing one: and each of these assertions is equivalent to one which asserts a duty--the first with an assertion that it is a duty to refrain, the second with an assertion that a positive action is a duty. But there is another distinction between some moral rules and others, which is of much greater importance than this one, and which does, I think, give a reason for thinking that the term "moral obligation" is actually used in different senses on different occasions.