I’m no philosopher, but…
So substance dualism is the view that, in addition to the material world governed by physics, there exists an immaterial world which presides over Agency (that is, free will) and Subjective Experience (muh qualia). Unlike Epiphenomenal Dualism, the more substantial brother asserts that the immaterial world really exists and has real effects on the material, and vice-versa. However, it operates under different laws which are not reductionist in nature, allowing Agents to be basic building-blocks of reality. This “explains” free will and qualia, by adding a bunch of undetectable ghosts that… have free will and qualia as basic properties.
Oh, and: the Immaterial world is completely undetectable except by its actions on human brains (none of which have ever been observed, but admittedly non-invasive scanning has poor resolution and brains in particle accelerators tend to stop making choices and having experiences fairly rapidly).
Now, the human spirit may be indivisible and immaterial, but the brain is not. And we know by now that the human brain is where actions in the material world come from: people’s mouths wouldn’t talk about their inner lives without muscles pulling around, which wouldn’t happen without nerve signals, which depend on other signals back into the dark recesses of the head. For substance dualism to be true, there must be some point, somewhere along the chain, where physics as we understand it from the physics lab is suspended and the immaterial world pushes some physical matter one way or another according to the will of that matter’s associated Agent.
This seems fine, provided there is some as-yet unknown mechanism by which this takes place. It is possible for the dualist to sidestep the charge that specific kinds of brain damage cause specific apparent changes to the Agent by supposing that the Agent has a message-sender that is exactly as complicated as the gaps in our understanding of the brain. That is to say, if we could selectively damage single cells and observe changes to the patient, then the Agent would need a message-sender as complicated as a cellular-level model of the brain, so that when that cell is damaged, the message can be corrupted in a corresponding way. For now we’re limited to very crude quantities of structural changes in the brain, so the dualist need only imagine that the Agent has a large handful of pieces of message-sending apparatus*.
But overall, this seems fine.
Except, there’s something a bit wrong here.
The human brain is fantastically complex . It could yet prove to be the case that its process are beyond our comprehension entirely, though I doubt that. In any case, it is without doubt a marvelous piece of gear.
But. It’s also a wet, squishy, warm lump of hydrogen, oxygen, carbon, and various other light elements. To a physicist, the difference between a brain and the same brain after random permutation via kitchen blender is not particularly huge. For this to be detecting new physics would be – it would be something beyond unprecedented. New physics a few centuries ago meant electricity, sure, and it wasn’t inconceivable that there were similar advances waiting to vindicate the notion of the brain as Immaterial-Uplink-device. But new physics today? After all those centuries of new physics moving outside of the chemical and biological domain?
Look. If you want to find new physics, you either need to do something very cold, very hot, or very regularly arranged in an unusual way (e.g. Quantum computers, Higgs bosons, Majorana fermions). The notion that that all human brains, regardless of the large structural differences between them, are being affected by forces that no other particle in any situation ever experiences, is not something you can seriously believe in the face of this vast gap. So you have a feeling of internal experience that you think can’t possibly be explained without a stuff-of-internal-experience in a non-physical realm. How would you quantify that feeling of inexplicability? A thousand to one? A million? The chances that your brain is doing something no physics experiment ever has is smaller than that, by quite a lot.
And even if we could find this new physics, the Immaterial is still made entirely of convenium. Isn’t it just – just silly to suppose that there are a bunch of Agents hanging out in paradox-space, who happen to vary among themselves in exactly the same ways that human brains vary among themselves? If you could look at the diverse variety of human behaviours, and the diverse variety of human brains, you could see that the two are the same. That the way brains vary, predicts exactly the way people vary. Isn’t it a bit much to suppose that on top of always and forever*** fitting exactly into the gaps in our knowledge of physics, the Immaterial also just so happens to always exactly produce the same predictions of the distribution of minds as its absence?
Why do we come with a laundry-list of cognitive biases? Because we’re brains built to approximate Agents, not Agents. Why do we have such a limited range of possible intelligence? Same reason. Why are willpower and sense perception, which one would think pretty integral to the free choice and qualia questions, so often damaged or broken by differences in the brain? Same reason Why does human variability look so exactly like the result of genetic variability and experiential variability? Same, simple, singular reason.
If you’re not a process implemented by a brain, what in the heavens are you? Make specific predictions. Tell me how to set up a combined Stern-Gerlach apparatus/MRI scanner or whatever so as to see when the soul pushes part of the brain around. There must be something you can do other than fiddling definitions around to make bad ideas sound cleverer.
* – Actually, we should expect to have brains that are the least complicated structures capable of picking up signals from the Immaterial, if we’re willing to accept things like the theory of evolution**. Which would seem to imply that the signals are at a minimum, as complicated as the entire brain – if they were simpler, we’d have simpler brains in turn (since the brain has no function except picking up the signal).
** – It’s possible for the history of evolution to be explained by, e.g., intermittent divine intervention, but it’s generally held that mutation/natural selection (or, as I have taken to calling it, mutation plus natural selection) is the best we’ve got.
*** – Maybe if physicists tomorrow said “OH, right, duh, obviously [X]. Well that solves everything, turned out there weren’t any souls in the Final Draft” then some substance dualists would change their minds. But merely watching the limit be approached – seeing how each door to extra physics affecting the brain has closed, with Primitive Type Agent Objects firmly on the outside – has not and will never have any effect. The notion that mere evidence can overcome an intuition is too strange.