In Darwinian Natural Right, one can see some movement towards David Hume's sentimentalist morality, based on the idea that moral judgment is an expression of moral sentiments or emotions. I continued to move in that direction in 1998 when I lectured at a conference on Edward Westermarck at the University of Helsinki, Finland. Westermarck helped me to see how a Darwinian science of evolution really does confirm the moral sentimentalism of Hume and Adam Smith, while refuting the moral rationalism of Immanuel Kant. The empirical testing of Westermarck's theory of the incest taboo seemed to show how a Darwinian moral sentimentalism could become an empirical science.
Over the past twenty years, this empirical moral science has been deepened by research in evolutionary psychology (people like Jonathan Haidt and Joshua Greene), experimental philosophy (people like Shaun Nichols and Joshua Knobe), and evolutionary anthropology (people like Robert Boyd and Joseph Henrich). The general conclusion that seems to emerge from this research is that the Humean sentimentalists are right, and the Kantian rationalists are wrong.
Recently, I have been thinking more about this while reading a book manuscript for Lexington Books entitled Evolution and the Foundations of Ethics. The author's project is to apply four different models of evolution to four metaethical theories and six normative ethical theories. The four models of evolution are those of Darwin, Richard Dawkins, Stephen Jay Gould, and John Haugh. The four metaethical theories are Error Theory, Expressivism, Moral Relativism, and Moral Realism. The six normative theories are Virtue Ethics, Natural Law Ethics, Social Contract Ethics, Utilitarian Ethics, Deontological Ethics, and the Ethics of Care. I appear in the chapter on Natural Law Ethics as someone who argues for a Darwinian theory of Thomistic natural law.
[Now, in 2017, I can identify the author as John Mizzoni. The book has now been published.]
Although there has been a lot of writing about applying evolutionary reasoning to contemporary moral philosophy, this book is, I believe, the only comprehensive study of all the various moral theories in the light of evolutionary science. I was disappointed, however, that the author never mentions Smith's Theory of Moral Sentiments, the writings of Westermarck, or the evolutionary moral anthropology of Henrich. The author mentions the experimental moral psychology of Greene only briefly in a few notes.
While I learned a lot from reading this book, the general conclusions that I draw from it are not the conclusions that the author wants to advance. The author argues that all of the moral theories considered in the book--the four metaethical theories and the six normative theories--can be seen as compatible with evolutionary science. I find that implausible, and most implausible of all is the author's claim that evolutionary reasoning about ethics can support Kantian deontological ethics. The author gives me no good reasons to doubt my previous conclusion that an evolutionary science of ethics proves that Humean sentimentalism is right, and Kantian rationalism is wrong.
The author points to the passage in The Descent of Man where Darwin quotes from Kant—“Duty! Wondrous thought . . . whence thy original?” Darwin promises to take up this question “exclusively from the side of natural history.” The quotation from Kant is from The Critique of Practical Reason (AA, p. 86). Immediately after this passage, Kant says that the rational grasp of the ought of pure duty shows us “man as belonging to two worlds”—the phenomenal world of natural causes and the noumenal world of human freedom transcending nature. Contrary to Kant, Darwin identifies the moral ought as a moral feeling: “Any instinct, permanently stronger or more enduring than another, gives rise to the feeling which we express by saying that it ought to be obeyed” (1871, vol. 2, p. 392). So Darwin’s study of morality “exclusively from the side of natural history” denies Kant’s “two worlds” view. Frances Cobbe saw this, and she denounced Darwin for rejecting Kant’s ethical theory. The author doesn’t explain how this can be compatible with the claim that Darwinian science supports Kant.
In Kant's Foundations of the Metaphysics of Morals, he declares: "reason of itself, independent of all experience, commands what ought to be done." Thus, "all moral precepts have their seat and origin entirely a priori in reason." Consequently, "the ground of obligation here must not be sought in the nature of man or in the circumstances in which he is placed, but sought a priori solely in the concepts of pure reason." Moral duty, then, must be derived by pure logic from a moral law that is stated as a rule of purely formal self-consistency. Moreover, this a priori reasoning of morality belongs to a realm of freedom that is outside the laws of nature. This is as far away from Darwin’s natural history of morality as one can go.
The author quotes Michael Ruse as concluding that the "spirit of Kantianism is antithetical to the spirit
of Darwinism." In the endnote or
this sentence, the author writes a long note on Joshua Greene’s argument that
experimental neuroscience refutes Kant.
As I have often argued, we could say that all natural law reasoning depends on hypothetical imperatives that have a "given/if/then" structure: Given what we know about the nature of human beings and the world in which they live, if we want to pursue happiness while living in society with each other, then we ought to adopt a social structure that conforms to human nature in promoting human happiness in society.
Natural law reasoning does not prohibit us from punishing any expression of a natural behavioral propensity. For example, pure psychopaths are probably expressing their biologically natural propensities. But given what we know about the harmful propensities of psychopaths, if we want to protect our society from harm, then we ought to punish psychopaths to protect ourselves from their harmful behavior.
Consider how this would apply, for example, to the moral and legal debate over homosexuality and gay marriage. Given what we know about the animal nature of homosexuality, if we want to pursue happiness, peace, and prosperity in societies with both heterosexual and homosexual individuals, then we ought to protect the liberty of homosexuals to live their lives as they wish, as long as they do not harm others. Consequently, the liberty of homosexuals would include the right to same-sex marriage, as long as we know that this is not harmful to others. That is the argument of Justice Kennedy's decision in Obergefell v. Hodges.
If one believes, however, as some of the opponents of same-sex marriage believe, that the governmental licensing of same-sex marriage will harm the children of same-sex parents, then natural law reasoning would condemn the legalization of same-sex marriage.
The debate here becomes an empirical question that will be settled by our reasoning about our experience: Is there any evidence that same-sex marriages harm children?