If I ought to do something, am I obliged to do it? And if I am obliged to do something, is it my duty to do it? I tend to assume the following principle, where A is an agent and X an act or rather act-type such as feed one's children.
P. Necessarily, A morally ought to X iff A is morally obligated to X iff A has a moral duty to X.
The necessity at stake is conceptual; so by my lights (P) is a conceptual truth. But, as if to illustrate that philosophers disagree about every bloody thing under the sun, a correspondent writes:
I don't see that "x is something I ought to do iff x is something I'm morally obligated to do" is a conceptual truth, or even true. [. . .] Non-consequentialist moralities allow room for good deeds that are not obligatory. If helping a stranger is a good deed and you are fully able to perform it without endangering others, then I am quite comfortable recommending to you that you ought to do it. But I am not suggesting you have any duty or an obligation to do so. [. . .] So, you ought to help does not imply you have a duty to help.
I will now try to show that you ought to help does indeed imply that you have a duty to help, assuming that one is not equivocating on 'ought' and is using 'ought' as it is used in (P).
I agree that there are good deeds that are not obligatory. Suppose my neighbor is away when an important-looking package is delivered to his door. I take it into my house for safekeeping until he returns. Surely I am under no obligation, moral or legal, to do such a thing. Yet it is a good deed.
But it does not follow that it is a deed that I ought to do, or that I have a duty to do; it is precisely a supererogatory action, one above and beyond the call of duty. (A supererogatory action can be something as trifling as this, and need not be grand or heroic, but more on this in a separate post on supererogation.) If I ought to X, and I omit to X, then I do something wrong. Therefore, if I ought to pick up my neighbor's package, but omit to do this, then I do something wrong. But obviously I do nothing wrong in leaving my neighbor's package where it lies. Hence it is not the case that I ought to pick up my neighbor's package. Nor do I have any duty to pick up my neighbor's package.
I suspect my correspondent is simply playing fast and loose with 'ought,' a word with several meanings in English. Some examples:
a. 'The car ought to start; I installed a new battery.' This looks to be a non-normative use of 'ought,' one with no relevance to moral theory.
b. 'If you want to get to Tucson from Phoenix by interstate highway, you ought to take I-10 East.' This sentence is a hypothetical imperative, and the subject-matter is morally indifferent.
c. 'If you want to be a successful hit man, then you ought to learn how to kill with a .22 caliber gun.' A second hypothetical imperative. Here the subject-matter is not morally indifferent, but the 'ought' has noting to do with a duty.
d. 'If helping a stranger is a good deed, and one wants to be helpful, then one ought to help.' Another hypothetical imperative, and close to what my correspondent said above. But this use of 'ought' is not the use in principle (P) above.
e. 'You ought to pay your debts.' A categorical imperative, and a morally relevant use of 'ought.' This is the use of 'ought' that is featured in (P) above.
In sum, (P) seems rock-solid and I will continue to adhere to it until someone can instruct me otherwise. But then I ask myself: Am I merely making precise how I shall use the relevant moral words? Is (P) above a merely precisifying, and thus partially stipulative, definition? If so, then ordinary language considerations won't tell against it.