In Plato’s day, naturalism about the mind-body problem was called the harmony account of the soul, i.e. mind arises from body like a melody from a string. Plato’s most striking argument against this is his last argument in the Phaedo:
And is the soul in agreement with the affections of the body? or is she at variance with them? For example, when the body is hot and thirsty, does not the soul incline us against drinking? and when the body is hungry, against eating? And this is only one instance out of ten thousand of the opposition of the soul to the things of the body.
But we have already acknowledged that the soul, being a harmony, can never utter a note at variance with the tensions and relaxations and vibrations and other affections of the strings out of which she is composed; she can only follow, she cannot lead them?
It must be so, he replied.
And yet do we not now discover the soul to be doing the exact opposite—leading the elements of which she is believed to be composed; almost always opposing and coercing them in all sorts of ways throughout life, sometimes more violently with the pains of medicine and gymnastic; then again more gently; now threatening, now admonishing the desires, passions, fears, as if talking to a thing which is not herself, as Homer in the Odyssee represents Odysseus doing in the words—
‘He beat his breast, and thus reproached his heart: Endure, my heart; far worse hast thou endured!’
Do you think that Homer wrote this under the idea that the soul is a harmony capable of being led by the affections of the body, and not rather of a nature which should lead and master them—herself a far diviner thing than any harmony?
Nagel’s defense of cognition in Mind and Cosmos helps to draw out the sophistication of this sort of argument, by arguing that an action made according to insight, theory, or a sense that something is right differs from an action with a purely physical story. None of us are surprised that hunger, fear, anxiety, lusts for pleasure and revenge, and other experiences like this have a physical component and an evolutionary justification, but it’s precisely this awareness that they have a physical component that makes it reasonable to step back from them and ask what role we want them to play in our behavior. But the awareness that some behavior is right, whether this is a behavior of belief in a theory or of how we should act is not like this. Insight into what is right – even where that insight is in fact imperfect or mistaken – is not the sort of thing that we can step back from and ask what role it ought to play in behavior. We suffer equally the experience of fear and the experience of the truth of an inference, but the first leaves the question open of what ought to be done or believed and the second does not. Any further action in response to the first needs some additional justification, which is ultimately (and usually immediately) some truth of the second kind.
Finding a physical motivation for belief always triggers the awareness that it is judged by another. As soon as we finds some physical basis for the belief that, say, all physical actions are demand physical contact we ask whether this is in fact the case, in the same way that the brute physicality of lust makes us immediately question whether it should be acted on. It’s not that the physical account debunks the belief, it simply forces us to raise the question of whether the belief ought to be followed or resisted in light of some sort of experience that is justified in itself, which is what moral education largely consists in. I spend most of my day trying to get kids to recognize or at least act on the sorts of reasons that are justified in themselves and can serve as a basis to critique to purely physical impulse to punch your sister, give up or slam doors in frustration, or eat the whole box of Cap’n Crunch.
It’s perhaps in this sense that there can be some justification for the idea that naturalism must be essentially immoral. It’s not so much that we need some sooper-dooper authority to ground something as mysterious as a moral imperative but that recognizing a purely physical basis of an action requires us to look for some standard that will allow us to incorporate it it into action. The “Euthyphro Problem” never arises. Naturalism doesn’t so much do away with a law giver as with the basis of moral education, because it demands that all experience share exactly that feature of rage, frustration, hunger, lust for sugar, etc. that forces us to judge them by a different kind of experience. Naturalism demands that the feeling of logical consistency or moral honor be just as physically motivated as hunger, but it is impossible to experience them in this way, and even if we could it would simply leave us looking around for a reason to be reasonable or a moral motivation for morality. Naturalism seems committed to the idea that there can be no such justification, though this leaves the Naturalist having to explain just what sort of claim naturalism is making about the world and what sort of theoretical and practical behavior it could consistently justify.