Bill R. wrote this new argument (to me at least) concerning moral realism and naturalism. I thought it was pretty interesting, especially in the way it clarifies the difficulty naturalism has in saying “this ethical duty matters to you,” so I’m posting it here for more visibility. The rest of what follows here is his.
Say, for the sake of argument, that d’s “that which we want most of all/for its own sake” is the same as the Christian concept of “natural ends.” Let’s call both concepts the summum bonum [ultimate or highest good], for convenience. Even under this assumption (which is obviously still being hotly debated), naturalism still does not support a universal morality, while Christianity does.
Under naturalism, is there any guarantee that this summum bonum (whatever it may be) is actually attainable? What if evolution has shaped us (forgive the anthropomorphizing) so that our summum bonum is forever just out of reach, like the proverbial carrot hanging by a string before the mule? An unsatisfiable desire — one that causes an organism to be always reaching, never resting — would seem to be a highly effective tool for motivating that organism to constantly seek more and more (babies, resources, etc.): It would also explain why there are so many people whose lives seem to be going well (including successful people at the top of their field), who nonetheless report dissatisfaction with their lives.
Under naturalism, love may get us a little closer to the summum bonum than cruelty, but what if the carrot is still forever out of reach? If I, as a rational agent, recognize that the summum bonum might be unattainable, but that the object of some lesser desire (say, to inflict pain on my fellow creatures) is easily attainable, why shouldn’t I settle for the latter? Why shouldn’t I choose to pursue, as my personal summum bonum, the best thing that I can actually get, rather than the best possible thing, which is probably unreachable? And if I have no reason not to choose a different summum bonum, then in what sense am I part of a naturalistic universal morality anymore?
If Christianity is true, however, then the summum bonum is God. For theism in general, there is no reason to expect that God is any more attainable than a naturalistic summum bonum. In fact, Christianity recognizes that it is impossible for humans to reach God on their own, through any religion. Christianity, however, is not a story about man finding God, but about God coming down and finding man. God, through the life and death of Jesus, guarantees that if we seek Him at the expense of everything else, we will find Him (thus attaining the summum bonum) — perhaps partially in this life, but completely in eternity.
Thus, even if a naturalistic summum bonum exists, then there is at least one crucial difference between naturalistic and Christian universal morality: the Christian, because of who God is and what He did on the Cross, can be certain of obtaining the summum bonum, whereas the naturalist has no such assurance. I’m not citing this difference as evidence that Christianity is true, of course. It just illustrates that the naturalist may have good reason to opt out of any naturalistic “universal” morality (making it not truly universal anymore), whereas the Christian who actually trusts God has no reason to settle for anything (or anyOne) less.