This Platonic longing is evident in the writing of Peter Singer. I was reminded of this in looking at the new edition, published this year, of Singer's The Expanding Circle: Ethics, Evolution, and Moral Progress. When this was first published in 1981, this was the first book-length response to the revival of Darwinian ethics initiated by Edward O. Wilson. It anticipated the agonizing struggle of philosophers over the past 30 years to come to terms with the new biological and evolutionary studies of morality.
One can see the influence of this book in Pinker's Better Angels of Our Nature, where Singer's idea of the "expanding circle" guided by the "escalator of reason" is a fundamental theme. But while Pinker emphasizes the role of reason in moral progress, just as Singer does, Pinker does not fully embrace Singer's Platonic/Kantian rationalism, because Pinker recognizes that pure reason by itself cannot move us to act without emotion or desire, and because he recognizes that whatever moral truths we discover are contingent truths about our evolved human nature, not eternal truths about the universe (see Better Angels, 642-50, 694-96).
The new edition of Expanding Circle has an "Afterword" in which Singer reluctantly admits that he has lost confidence in his earlier Platonism, although he still can't bring himself to give it up.
In the original book, Singer insists: "there is something in ethics which is eternal and universal, not dependent on the existence of human beings or other creatures with preferences. The process of reasoning we have been discussing is eternal and universal. That one's interests are one among many sets of interests, no more important than the similar interests of others, is a conclusion that, in principle, any rational being can come to see" (105-106).
But then he immediately adds some comments that show his ambivalence about this. He notes that it was easy to believe in "moral laws which exist independently of us" when those laws were believed to have been handed down by God. But now--for people like Singer--this religious belief is impossible. "And the more we think about what it could mean--outside of a religious framework--for there to be eternal moral truths existing independently of living creatures, the more mysterious it becomes" (106-107).
This is "mysterious," because, as John Mackie argued, this implies some very queer metaphysical entities in the universe, something like Plato's Idea of the Good. "How can there be something in the universe, existing entirely independently of us and of our aims, desires, and interests, which provides us with reasons for acting in certain ways?" (107).
So, in the original book, Singer left his reader wondering whether he really believed in cosmic moral truths that were "eternal and universal," or whether he regarded such a cosmic morality as too "mysterious" to be credible.
In his new "Afterword," Singer points to this part of his book as showing "how ambivalent I was about the idea of ethics being objectively true and rationally based" (198). Now, he admits:
I no longer believe that this argument succeeds. The judgment that "one's own interests are one among many sets of interests" can be accepted as a descriptive claim about our situation in the world, but to add that one's own interests are "no more important than the similar interests of others" is to make a normative claim. If I deny that normative claims can be true or false, then I cannot assert that this claim is true. It too could be treated as just one preference among others--except that now there is no basis for saying that we ought to maximize the satisfaction of preferences. . . . The denial of objective truth in ethics thus leads not, as I had tried to argue, to preference utilitarianism as a kind of metaphysically unproblematic default position, but to skepticism about the possibility of reaching any meaningful conclusions at all about what we ought to do. The only conclusions we could reach would be subjective ones, based on our desires or preferences, and therefore not ones that others with different desires or preferences would have any reason to accept. I was reluctant to embrace such skeptical or subjectivist views in 1981, and that reluctance has not abated over the intervening years. (199-200)But then, after making this admission, he concludes in the last sentence of the "Afterword" with an appeal to "the existence of objective moral truths" (204).
One can see this longing for a moral cosmology of eternal and universal truths in Plato's dialogues. But a careful reading of the dialogues also suggests that while Plato and Plato's Socrates thought such a moral cosmology might be necessary for the good morals of most people, it was not really plausible, because the universe appears to be morally indifferent. The universe does not care for or about us. But we care for ourselves. And that care for ourselves as an expression of our human nature constitutes the natural ground for the human good as perfected in the moral and intellectual virtues.
Darwinian science provides an evolutionary account of life, including human life, that supports such a naturalistic ethics of human care. The denial of moral cosmology does not drive us into nihilism, because the evolved nature of human beings as caring for themselves constitutes a natural ground for the human good, even though the human species is not eternal or invariant.
These points have been more fully elaborated in some previous posts, which can be found here, here, here, here, here, here, here, and here.