system
Given the following post, respond as if you're commenting on Reddit:
human
Who cares if moral realism is true?

I've never seen this assumption formalized in the literature, but it seems to me that there is a general feeling that it is "important" whether moral realism is true or not.  Without being overly-general about the personal values of individual philosophers, the potential truth of moral realism seems to carry more weight than the potential truth of, say, some obscure and technical mereological theory:  that is, if moral realism is true, then we expect it to have a significant impact on how we view humans and human action, and we expect it to have an appreciable impact on our own behavior.

Upon further analysis, however, I'm not convinced that this position is correct.  Suppose that at least some moral facts are true, and that humans are capable of learning the truth of these facts.  Why should these facts alone influence anyone's behavior, in any situation?  It may be the case that the true correct theory of morality entails that if an agent does X and X is wrong, then that agent will be harmed, and that constitutes a good argument for why you should not do X; but if the set of actions that are morally wrong is just a subset of the actions that will harm you, then shouldn't we just dispense with trying to find a metaphysical account of moral properties and simply focus on describing the actions which are personally/socially harmful?  The addition of a moral property adds nothing; people can only be compelled to act by physical properties.  Someone may decide that they want to act in accordance with moral properties, but this decision seems arbitrary.

I suppose I'm getting at the oft-repeated thesis that moral facts must be causally inert, but instead of using this as an argument against moral realism, I'm simply pointing out that this means we shouldn't really care about moral realism.  Knowing that an action is wrong or right seems to be as irrelevant as knowing that the action is occurring X miles from the sun.  Now, you could certainly still be interested in whether moral properties exist or not for purely intellectual reasons, but as I pointed out in the beginning, I don't think that people are interested in moral realism for purely intellectual reasons.  They want something more out of it.

To sum up:  should the truth or falsity of moral realism affect my behavior, and how?  Is it possible for moral facts to be causally efficacious?
gpt
The view that moral facts would necessarily affect behavior is called motivational internalism. Basically, just by learning some moral fact, you automatically have a motive, though not necessarily overriding, toward some end. Most moral realists reject this view because it seems impossible for any facts to behave like this.

A more developed view, reasons internalism, states that learning some moral fact necessarily gives you a reason to act in some way. Giving you a reason to act perhaps (on some moral realists' accounts) gives you a motive to act insofar as you are rational.

If moral realism and reasons internalism are true, then moral facts should affect your behavior so long as you are rational. The question whether these are true is important because the answer shows whether we have the authority to determine our own reasons, or there is some independent authority that gives us reasons (with the stipulation that some, like Korsgaard, think we have the authority to determine our own reasons but rationality is such that we necessarily give ourselves moral reasons).