A friend of mine is an emotivist while I am a moral realist. When talking with him I often make arguments of the following form, and I'm curious how other emotivists, moral relativists, or indeed other moral realists would respond.

The root claim is that moral systems may be loosely thought of as axiom systems (albeit only formal in a weak sense). Man possesses senses that provide information about (both Platonic and real, although a Platonist might say I'm repeating myself) objects. If one's sense of what is right and wrong is also providing information about real objects, then we shouldn't find it surprising that some moral systems are more "effective" (in the "Unreasonable Effectiveness of Mathematics in the Natural Sciences" sense of the word) than others.

Just as physics provided a useful proving ground for various mathematical formalisms of physical phenomena, I think that bioethics provides an analogous proving ground for various moral systems. A hot-button but proximal limiting case is abortion. Here, poorly formulated notions of what a person is or is not, what rights a person does or does not have, etc. are quickly broken, no matter what side of the issue they stand on.

A common objection to this line of reasoning I've heard is that not everyone, even people formed in equally "advanced"/"developed"/etc. cultures, agree on what is right or what is wrong. However, by analogy with other formal systems of thought, we should not be surprised by this. There are plenty of historical examples where, among equally "advanced" societies, some reached conclusions that were obviously absurd to those who would ultimately be proven correct. We should expect to find:

Unequal advancement across different areas. Some societies developed the wheel before writing, some writing before the wheel. Analogous things have occurred in societies' moral development.
Very serious investment in very wrong ideas. Nazi Germany pointed infrared telescopes and even fired rockets straight upwards from Northern Africa. Based on their assumption that they were living on the interior of a hollow Earth, they intended to target New York City.
Formal systems modeling real objects sometimes reveal surprising, counterintuitive conclusions. These are sometimes rejected by the majority of practitioners, despite ultimately being proven correct. The history of physics has no shortage of events like this (the ultraviolet catastrophe, quantum mechanics, etc.) A loosely analogous event might be Pope St. Paul VI arguing in his encyclical Humanae Vitae that even novel chemical forms of contraception are morally repugnant. This shocked and offended his contemporaries, but later moral theologians would come to agree with his conclusions.

Despite the increasing number of anti-Western takes on this issue, the most "effective" moral system is clearly the one entailed by the Western Christian meme complex. However, one disagreeing with this could probably make arguments analogous to mine. So to emotivists/non-cognitivists/anti-realists: how precisely do we know that moral realists are not using reasoning to make predictions about how their senses will respond to moral objects?

(I am not a philosopher or theologian by trade, so rip my misunderstandings to shreds)

moral philosophy

A Sketch of Moral Realism