1. M is the moral thing for me to do, and M is not in my best interest. (premise)
2. M is the moral thing for me to do. (1)
3. M is not in my best interest. (1)
4. It's the case that agent A ought to do X iff X is in A's best interest. (premise)
5. It's not the case that I ought to do M. (3 and 4)
6. It's the case that X is the moral thing for A to do iff A ought to do X. (premise)
7. It's not the case that M is the moral thing for me to do. (5 and 6)
Yikes! Something has gone wrong here, since (2) and (7) are in direct contradiction.
Denying the (Possibility of the) Premise in (1)
This is the route taken by some theists who believe it is essential to morality that God will step in and make it be the case that acting morally is always (eventually) in our best interest, and vice-versa. Usually by means of a reward and punishment afterlife. Reincarnation schemes can work too.
I positively affirm that sometimes acting morally is not in one's best interest now or later, so I'll need to deny another premise.
Denying the Premise in (4)
This premise equates what one 'ought' to do with what is in one's best interest. But 'ought' in what sense? I maintain that 'ought' always takes an end, which might be in order to fulfill one's own best interest, or in order to comply with the law, or in order to improve the happiness of society, etc.
So I do deny the premise in (4), unless the end involved is something like in order to fulfill one's own best interest.
Denying the Premise in (6)
Finally, this premise equates what one 'ought' to do with what's the moral thing for one to do. I give the same answer as before: it will depend on which end the 'ought' takes.
I deny the premise in (6), unless the end is a moral end. Which ends are moral ends? No need to answer that in general now. There's a more specific issue at hand:
Is in order to fulfill one's own best interest a moral end?Answering 'yes' to this would supply one of the missing premises which would then force me to deny the possibility of (1), even with my end-relational view of 'ought.'1 But I answer 'no' for the same reason I affirm the possibility of (1): I maintain that moral 'oughts' can be true even against an agent's own best interest.
Saving a drowning child is still what I morally ought to do, even if I don't have a guarantee that my own best interest is thereby promoted. I invite opponents to openly disagree with this.
Wait a minute, why not just give the drowning child example up front instead of going through the whole argument analysis rigmarole? I wanted to show that affirming (1) doesn't lead to a contradiction. It would also take affirming (4) and (6), which is consistent with denying the possibility of (1) but isn't forced by affirming (1).
Simply put, I can separate my own best interest from moral rightness...if I consistently distinguish my own best interest from moral rightness. I do so with the crazy notion that the moral thing for me to do has to do with the best interests of everyone affected, without privileging myself.
1. We would have to add that fulfilling one moral end gives the same result as fulfilling moral ends in general. The 'ought' in (4) would also need to be specified as having the best interest end I mentioned.