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How Does Dualism Understand Personal Identity?

Both neurosurgeon Michael Egnor and theology professor Joshua Harris acknowledge weaknesses in their philosophies’ understanding of personal identity

In “The Body and the Soul” podcast, neurosurgeon Michael Egnor interviews theology professor Joshua Farris on how a sense of personal identity is preserved (or not) in Aristotelian vs. Cartesian philosophy (both are dualist philosophies; they do not think that the mind is merely a product of the brain). Along the way, Michael Egnor talks about the remarkable way that neuroscience affirms a dualist view.

A partial transcript and notes follow:

Michael Egnor: Had it not been for neuroscience, which led me to a Thomist view, I would probably be a Cartesian because I do agree that there’s a great deal to say for it. Although my sense of Cartesianism is that the closer we get to Berkeley and idealism, the better Cartesianism gets…

Michael Egnor

Whatever the body–mind–soul relationship, I think we would agree that it needs to fit in as a coherent whole with nature. I’m not a naturalist, but we are obviously a part of nature in a very meaningful way. I think idealism is very nice that way in that you can get a consistent, coherent metaphysical perspective from a Berkeleyan metaphysical way of looking at things. And you can do the same with Thomism I don’t think you can do it with Cartesianism.

Joshua Farris: So let me ask you some, a couple questions. My reticence to move in the direction of a sort of Berkeleyan or — I guess you might call it — Cartesian idealist view that has a Berkeley inflection to it is that Berkeley doesn’t specify, from what I recall reading, in a robust way, the sort of individual essence or individuality or particularity that just naturally comes out of a Cartesian way of approaching these issues…

In neuroscience you have these split brain experiments, right? And you have evidence that suggests that there are split perspectives that emerge as a result of the split brain and you have other neuroscientific experiments that suggest similar phenomena, which seems to support something like a more robust substantial dualism that is not had… on maybe an idealism…

Michael Egnor: Right? I think the strongest argument from science for idealism is an observation that I heard about years ago in college that still fascinates me. And that is when you look at the world on the quantum mechanical level, matter disappears. That is, in its most basic, detailed reality at the quantum world, nature is an idea; it’s not material.

Joshua Farris

Electrons are not little balls of things. Electrons are ideas. They’re, ideas expressed by equations. For example, when you look in a reference book and it gives you the mass of an electron, it doesn’t say which electron. If you want to know the mass of a billiard ball, you have to say which one, because they’ll be a little different from [each other]. But there isn’t any difference from electron to electron. There’s one mass. People have even said, do we know that there is more than one electron? Could it just be one that is popping up everywhere?

So the whole notion of individuation of matter disappears at the quantum level, which is a very idealistic way of looking at the world. I think quantum mechanics is sort of the scientific expression of idealism, and it’s a powerful argument. So you really can make a case that matter is in the mind because quantum mechanics is all mind.

From that perspective, I think idealism is true. And I think it is, in some ways, the best way of looking at nature. But there are aspects particularly of the mind–brain relationship that strongly support it. You mentioned split-brain surgery, which is endlessly fascinating stuff. The original research on it was by Roger Sperry, a neurophysiologist who worked in the mid 20th century and won the Nobel prize for this.

I’ve operated on and worked with split brain patients over the years… Sperry found perceptual disconnections that were very subtle, very difficult to find. That’s why he won the Nobel prize; they weren’t obvious. It took a lot of very subtle research to find it. The most remarkable thing about these people is that they’re no different after the surgery than they are before the hemispheres of their brains are functionally disconnected. And they’re the same person.

It would be as if you took your chainsaw to your desktop computer, cut it in half and it still worked just fine…

And that’s an awfully strange thing. It was so strange that it led to Sperry to reject materialism. He had no use for the materialist view at all. What splits is only perception. But intellect doesn’t split, a sense of self doesn’t split, the will doesn’t split. And there’s been fascinating follow up on, Sperry’s work by two researchers, Justine Sergent and Yar Pinto, who have looked at these patients more carefully.

Aristotle (384– 322 BC)

There’s a brilliant experiment that Sergent did with these patients. She took a bunch of split brain people, and presented letters to their visual fields, presenting different letters to the isolated hemispheres. Like your right hemisphere might see a K and your left hemisphere might see an N.

Your hands and your arms, of course, are controlled by the opposite hemisphere. So in a person with a split brain, the right hand is controlled by the left hemisphere, which sees the right visual field. So the right hand can only respond to the right visual field. And the left hand can only respond to the left visual field. And there’s no connection — at least no obvious connection — between the two of them.

So she would show them letters and ask them to, push a button when they saw a letter. And then she’d say, I want you to push a button when one or both of the letters are vowels. So people would see these disconnected letters and the different hemisphere aren’t connected. And when they would see a vowel, they’d push it.

But they weren’t told which hand to use to push the button. There was a button at each hand. As it turned out, when they would see a vowel, say, in their left hemisphere, they would just as often push the button with their left hand as they would with their right hand. They pushed the buttons regardless of which hemisphere was driving the hand. It was just 50-50, which meant that somehow the hemisphere that didn’t see the vowel knew it was a vowel.

The interesting thing is that these people still had a perceptual disconnection but they could figure out which one was the vowel. That was not disconnected. That was unitary. It didn’t matter which hand and which hemisphere knew. That so beautifully fits the Aristotelian to view of the rational soul that it takes my breath away.

What it’s saying is that the perceptual disconnection is there because the sensitive soul, which is the material powers of the brain, is split. The sensitive powers of the soul are split so perception is split. But intellect and will, which are immaterial powers of the soul, cannot be split. And indeed they are not split with split brain surgery. So it’s beautiful work it’s fascinating.

René Descartes (1596– 1650)

It hews perfectly to the Aristotelian/Thomistic model of the mind–brain relationship and Cartesianism doesn’t explain it very well. Idealism doesn’t explain it well either but Thomas [Aquinas] nails it.

The other thing that I think is absolutely fascinating… is an observation by Wilder Penfield, the pioneer in seizure surgery back in the 20th century.

Penfield noted that there are no intellectual seizures. That is, that when people have seizures, the seizure is a kind of random stochastic activation of the brain. Electrical impulses get going and they can happen anywhere, do anything. It can make your arm jerk. It can make you fall down unconscious. It can make you see flashes of light. It can make you have emotional experiences. It can make you have memories and smells… except people never have intellectual content.

People never think abstractly during a seizure and that’s remarkable. No one ever does calculus as a part of a seizure or even simple arithmetic. No one ever adds one plus one repeatedly as a seizure, no one ever contemplates justice or mercy as a seizure. But practically anything else you can think of has been described… and Penfield said, “Why not?”

If most of the brain is devoted to abstract thought, why wouldn’t an occasional seizure fire off an occasional abstract thought — and it never does. And that’s exactly what Aristotle would’ve said: Abstract thought is not material doesn’t come from the brain. The brain conditions our ability to think. Abstractly, if you drink a lot of alcohol and you have ethyl floating around your neurons, you’re not going to think abstractly as well as if you don’t. But the actual cause of the abstract thought is not the brain. It doesn’t come from the brain.

But then again, there are thoughts that are caused by the brain. But they’re not abstract; they’re emotions, they’re perceptions, they are movements. So this dichotomy between perception and cognition is very real in neuroscience and the only metaphysical framework that also has that dichotomy is the Aristotelian/Thomistic understanding of the soul.

Joshua Farris: Yeah. I guess I’m hung up on the Aristotelian framework as being the only way to make sense of or explain that sort of data. I mean, Richard Swinburne, in his The Evolution of the Soul, gives all sorts of thought experiments about how desires and perceptions are somehow functionally integrated processes that are dependent upon cognition and neurology. So he recognizes that. So in that way he may be affirming something like an Aristotelian view of the body but affirms let’s say, hypothetically, emergentism as a phenomenon that is set up by God as a sort of law-like relationship. There is this lawful occurrence when these complex set of conditions are met. Would that not provide any sort of explanatory power similar to the Aristotelian conception?

Michael Egnor: Sure. I say this humorously, but I actually believe it. First of all I think idealism is best understood as Augustine said, that “Creation is a thought in the mind of God.” We and the universe we inhabit our thoughts in God’s mind. They’re quite real because they’re thoughts in God’s mind… That’s an idealistic under understanding of metaphysics. But I would say, tongue-in-cheek, that God is a Thomist. That is that the structure in the divine mind hews rather closely to the Thomist view. So you can say that I’m a Thomist and idealist…

My problem with the Cartesian view is that Cartesian metaphysics is so raw in so many ways… I can’t accept the mind–body metaphysical aspect of such an inadequate metaphysics. In so many other ways, its just great.

For example, what is matter? That is, that if one comes from a Cartesian perspective how does one explain? I mean, what is matter?

Joshua Farris: Yeah, well, obviously there’s the traditional Descartes sort of line or at least the interpretation of Descartes and there’s the Neo-Cartesians who don’t always put their full commitment behind that sort of definition.

Michael Egnor: And the definition would be “that which is extended in space.”

Joshua Farris: Yeah. It’s a sort of quantitative measured extension; that’s what matter is. But again, a contemporary Cartesian isn’t committed to that necessarily. I don’t think it follows from the commitment that one makes about the soul or personal identity. I am wondering if something like an Aristotelian conception of matter or an idealistic slant or a sort of Aristotelian conception of matter can be compatible with a Cartesian view of the soul. I mean, most Cartesians today are actually affirming. They’re not coming with a sort of full blown metaphysical picture. They’re not coming at it with the sort of the freight of Aristotelian chronological categories but they are gesturing maybe in that direction.

Michael Egnor: The problem is that in the Aristotelian view of matter, … in a most fundamental way matter is potency. But matter, I think Aristotle would say, is the principle of individuation. And in Cartesianism, at least for a human being, the principle of individuation is the soul. It’s completely different so I don’t see how you can blend them.

The Aristotelian understanding of matter is that it individuates; his understanding of form is that it doesn’t individuate. It’s the principle of intelligibility. It’s not the principle of individuation. So in a sense, the Aristotelian view of the human person would have just the opposite metaphysical commitment to that of Descartes, that it’s the matter of the person that individuates the person and the soul is the intelligible part. Descartes would say, “Well, the matter is measurable. So the intelligible part and the soul is what individuals it’s kind of the opposite.”

Joshua Farris: That’s right. If you come at identity from an Aristotelian perspective, if matter is what individuates, then I don’t think we’re ever going to get at that more fundamental feature. And I think that’s the harder problem.

I think there’s probably on logical ways we could make sense of how matter works and how it affects the mind, how it affects perception. We can make sense of that if we have a sort of functionally integrated soul–body interaction but I don’t think the Aristotelian can ever make sense of the individuality of personhood.

Michael Egnor: Right, and I think the Aristotelian almost would try to skate over that by saying that the person is the composite. So the individuation of the person is because of his matter, but the person himself is the composite. Therefore he is individuated because he’s composite of matter. I do agree that that’s kind of skating. I don’t come away emotionally satisfied with that because there is, let’s face it, metaphysically simple me. And I was going to say, “No I don’t know me well at all. Me is what knows not what is known.” … I do agree that the Cartesian view can handle that. And I still keep going to the idea, well, if we were thoughts in God’s mind and God was a Thomist maybe that would handle it well.

Next: Philosophy of science and philosophy of religion

Here are all the segments in order:

A neurosurgeon and a philosopher debate mind vs. body. Philosopher Joshua Farris defends controversial Cartesian dualism. Neurosurgeon Michael Egnor critiques it but thinks it may account for near-death experiences. They both critique emergentism, the view that the mind, while not merely what the brain does, emerges from the brain and has no separate existence or origin.

How does dualism understand personal identity? Both neurosurgeon Michael Egnor and theology professor Joshua Harris acknowledge weaknesses in their philosophies’ understanding of personal identity. Aristotelianism, in Egnor’s view, interprets the mind– brain relationship better but Cartesianism, in Harris’s view, interprets personal identity better.

Excluding all reference to God from science is a form of theology. It’s negative theology, to be sure, Michael Egnor and his guest Joshua Farris agree, but still a theology — and one with implications. The neurosurgeon and philosopher agree: Excluding God from science provides an opportunity to make up all sorts of illogical ideas and call them science.

You may also wish to read:

How did Descartes come to make such a mess of dualism? Mathematician René Descartes strictly separated mind and matter in a way that left the mind very vulnerable. After Descartes started the idea that only minds have experiences, materialist philosophers dispensed with mind, then puzzled over how matter has experiences.


Dualism is best option for understanding the mind and the brain. Theories that attempt to show that the mind does not really exist clearly don’t work and never did. Neurosurgeon Michael Egnor reviews the mind-brain theories for East Meets West: Theology Unleashed. He think dualism makes the best sense of the evidence.

Show Notes

  • 00:10 | Introducing Joshua Farris
  • 01:17 | Blended Thomist Cartesianism
  • 08:06 | Berkeley and Idealist Views
  • 09:23 | Quantum Mechanics as an Expression of Idealism
  • 11:27 | Split Brain Surgery
  • 16:18 | Seizures and the Brain
  • 25:21 | The Aristotelian View

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How Does Dualism Understand Personal Identity?