What happens, morally speaking, if I promise to do something that happens to be slightly immoral? Do I still have some kind of obligation to do it?

Nice question!

On some views, there's no judging intrinsically whether doing what you promised is immoral—slightly or otherwise. If you're a consequentialist (someone who thinks consequences always decide what's right), the question is what, overall, produces the best consequences, and it might be that overall, it's better to do what you promised, even if it's something we'd normally expect you shouldn't do.

Someone else could say that the case contains a moral dilemma by its very nature. One the one hand, someone might say, it's wrong to break a promise. On the other hand, we've assumed that what you've promised to do is also wrong. On that way of looking at it, we have a dilemma, and on one way of understanding dilemmas, you will do wrong no matter what. That said, you may still be obliged, all things considered, to do what you promised—or not to, depending on the case.

We could add other theoretical possibilities here, but for anyone who faces a situation like this in real life, the answer is "It depends." For example: suppose that I've promised to lie to someone (who's not up to anything bad). The basis for promising is that by lying, I'll keep something really bad from happening. You don't have to be a consequentialist to think that sometimes consequences carry the day.

I'll confess to being a bit suspicious of answers that come from applying a moral theory. That's because I doubt that moral theories are likely to be as subtle as careful, informed moral judgment. I have some sympathy with the sort of answer a virtue ethicist would give: the right thing to do here is what a virtuous person, who fully grasps the circumstances, would do. That's not to tie myself to "virtue theory" but it is to say that I doubt that there are any good algorithms for settling hard moral cases. Seems to me that depending on the case, it might be right to keep the promise, or it might be wrong, and apart from knowing a lot more there's no good way to say.

I think a lot hinges in your question on the word "slightly". Is there a moral obligation to keep a promise to do something that is "slightly immoral"? I think that the answer has to be "No", since the value of duty to keep promises is not in question, and the act contemplated is only "slightly" immoral. OK, but how slightly? Would it help if you had written, "if I promise to do something that is utterly and completely immoral"? Or if you had written, "If I promise to do something that is only ever so slightly, just the teeniest barely discernible bit, immoral"? I think such gradations make a big difference, and it is not very clear how "slight" the immorality has to be before it ceases to conflict with the important general obligation to keep promises. Of course much depends also on the question to whom the promise was give, why, under what circumstances, and so on. These all need spelling out before we can address the question with any hope of answering it.

Read another response by Allen Stairs, Jonathan Westphal
Read another response about Ethics