A Quick Argument Against Nontheistic Moral Realism

A brief meditation on the eternal Moral Law written on our hearts

Nontheistic moral realists often claim that moral truths such as "it's wrong to kill for fun" is an objectively binding brute moral fact independent of human whims similar to 2 + 2 = 4.

But this positions falls apart quickly once we attempt to examine where these facts come from and why anyone would think we are strictly normatively bound to follow these rules.

Nontheists will say that these facts are grounded in, for example, objective facts about what's good for the human species and our cooperation amongst each other. But these naturalistic facts are obviously contingent, and could have evolved differently.

Suppose 1 million years ago the hominid species was geographically isolated into two separate tribes. We could imagine a world where one of the tribes develops a special cultural ritual wherein their reproductive success is maximized only when they murder the members of the other tribe over scarce resources.

Indeed, there's no barrier to think that naturalistic evolution could have programmed each tribe to have a basic aggressive war-drive that contributes to maximizing reproductive fitness by murdering rival tribes but killing members within your own tribe would diminish fitness.

Under the naturalistic account of morality, why would having an aggressive war-drive not be considered morally good?

So, in this world, it'd be hard to come up with a truly universal, normatively binding moral law that says it's always wrong to kill innocent humans instead of merely a law that says it's wrong to kill innocent humans within your own tribe.

Furthermore, a purely naturalistic worldview that says the rise of human beings is a mere mechanistic cosmic accident provides zero rational basis for thinking that human life has ultimate intrinsic value. Indeed, if moral facts can just be completely "brute" with no underlying rationale, then how do we know it's not a brute moral fact that only bacteria have intrinsic value but humans do not?

By what normative authority do we appeal to say that's not a brute moral fact? And if you appeal to human reason, the evolutionary fallacy arises again: why expect human reason to be anything other than a contingent and fallible faculty biased by tribal allegiances instead of a faculty capable of discerning a universal and eternal Moral Law that's binding for all human beings?

It is only because God created humans in His image and put His eternal Moral Law into our hearts that we find the intuitive appeal of moral realism appealing in the first place.