Some people argue that moral obligations can be grounded in scientifically verifiable facts about human wellbeing and flourishing. This view is a form of ethical naturalism. For these people moral rightness is just the property of promoting or enhancing human flourishing.
Plato refuted this argument over 2,000 years ago in his famous dialogue The Euthyphro. The Euthyphro argument is commonly appropriated in the form of a dilemma: “is an action right because it promotes human flourishing or does it promote human flourishing because it is right?
If ethical naturalists take the second horn of this dilemma and claim that something promotes human flourishing because it is right then things are right prior to, and hence independently of, whether they promote human welfare; so the ethical naturalists’ position here is false.
If the ethical naturalists take the former horn then morality is arbitrary. If rape or murder or cruelty for fun had the natural property of promoting happiness then rape and murder and cruelty for fun would be morally required but it is impossible for these things to be morally required; so ethical naturalism is clearly absurd.
Moreover, if things are right because they have natural properties, like promoting human flourishing, then one cannot meaningfully say that human flourishing is good. To say “human flourishing” is good is just to say that ”human flourishing is human flourishing,” which is just an empty tautology.
So in short, Plato’s Euthyphro dilemma refuted ethical naturalism 2,000 years ago. Contemporary ethical naturalists who say otherwise just have not read Plato and have never come across this argument before.
In any event, ethical naturalism is deeply problematic. Throughout history people have appealed to human flourishing and forms of ethical naturalism to justify atrocities. The Inquisition, for example, was often justified by Dominican Theologians on the basis that it was in accord with natural law, and natural law was understood to be grounded in human flourishing. Similarly, wars have been justified on the basis of moral theories based on human flourishing. Stalin and Lenin wanted to bring about human flourishing through a communist utopia and murdered millions in their pursuit of that. And the Americans dropped an atomic bomb on Japan because they wanted to promote the flourishing of American soldiers that were going to invade.
Finally, it is obviously insulting to suggest that right and wrong are grounded in scientifically verifiable properties like this. There are a large number of people who live morally upright lives who do not believe in ethical naturalism. The suggestion that these people are all immoral is obviously false. Name me one moral action a person who is not an ethical naturalist cannot do? This unanswerable question shows that ethical naturalism is clearly false.
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