Russ Schafer-Landau, in "Ethics as Philosophy: A Defense of Ethical Nonnaturalism," sets out to defend moral realism against a number of criticisms.
His primary, positive argument is as follows:
1. Ethics is a species of inquiry; philosophy is its genus.While I find this argument to be logical, I still feel that something is wrong with it. I think all he has managed to do is prove moral realism is possible, not that it's necessarily true. I think the problem here is with premise 3 (which he acknowledges is likely to be the premise most contested). I think philosophical truths have a possible realistic status, not necessarily a flat-out realistic status. I think it’s possible to philosophize about entirely non-real things in a manner that is coherent and rational, and if this is true then his argument does nothing to prove the validity of moral realism, only to prove that it is possible with the bounds of this argument. Ancient Greek philosophers disagreed upon the natural substance of things; some claimed it was water, others fire, or some mysterious Apeiron. They all seemed to be making rational arguments on why they thought each substance to be the foundation, and yet today we believe them all to be wrong. If it is possible to philosophize about non-real things, then to say that ethics is a species of philosophy does not do anything to prove its realness. While I think his arguments against moral disagreement and the causal inefficacy of moral facts are effective for dismissing those arguments, I think the argument still stands simply that entirely rational arguments within philosophy can be made about non-real things, and thus his general argument does not prove moral realism to be true.
2. A species inherits the essential traits of its genus.
3. One essential trait of philosophy is the realistic status of its truths.
4. Therefore moral realism is true.