It seems self-evident that one has most reason to always act in one’s own interest, and it seems self-evident that one has most reason to always do one’s duty; but both principles can’t be right if there are ever situations where one’s duty conflicts with one’s interest. Famously, Sidgwick thought that, in a case of conflict, there was no reason to prefer acting in one’s own interest to doing one’s duty. This problem, called “the dualism of practical reason,” was thought by Sidgwick to pose a great difficulty for the foundations of ethics.
A way out of the problem is to invoke some hypothesis that makes interest and duty align. For example, many religions propose that if a person always does his duty, he will be amply rewarded in the afterlife. Though doubtful that God’s waiting around to do this, Sidgwick believes that this would be a way around the dualism of practical reason.
A cute little transcendental argument lurks:
Since it is self-evident that I have most reason to do what is in my own interest, and it is self-evident that I have most reason to do my duty, both must be true. The only half plausible way for these two to align is if God properly awards duty following. So, the only half plausible conclusion is that God properly awards duty following. Hence, if my duty and my (non-supernaturally affected) interest seem unaligned, it would be best to do my duty.
This argument is certainly open to other objections, but here’s a particularly entertaining one. God could make sure that one’s duty and one’s interest are aligned by other means. For example, if one’s duty is always to maximize the total well-being of all sentient life, just suppose that whenever anyone acts in his own interest, God sees to it that everyone else’s well-being is astronomically increased. Provided that the benefit God provides to others is sufficiently significant, there is no conflict between one’s duty and one’s own interest.
It might be replied that this divine coordination scheme is less plausible than the standard one. But, if it really isn’t true that I have more reason to do my duty than act in my own interest, it’s tough to see why. So maybe this objection could be overcome if the defender of the transcendental argument will work out the theology for us, but I’m not going to cross my fingers.
Oh, and why do we tend to like the reward-style divine coordination scheme the best, anyway?