Future Science of Brain Surgery and Substance Dualism

An argument for substance dualism recently appeared on Cameron Bertuzzi’s Youtube Channel, “Capturing Christianity”. The argument itself goes back at least to Richard Swinburne. As far as I can tell, he came up with the first version of this argument. You can see the presentation on Cameron’s channel here, or you can read a detailed presentation from Swinburne, R., 2013. Mind, brain, and free will . Oxford University Press.

I’m not interested here in discussing the formal argument itself, but rather a thought experiment that the argument utilizes. The thought experiment goes like this.

  1. People can lose one hemisphere of their brain without their personality being significantly affected. At the very least, they retain their self-identity.
  2. It will be possible in the not-too-distant future to remove one hemisphere of a person’s brain and transplant it into the body of another person.
  3. At this time, maybe someone removes both hemispheres of a patient’s brain and places one hemisphere in one body and the other hemisphere in the other body.
  4. This raises a question. Which of the resulting people is me? The one with my left hemisphere, the one with my right hemisphere? Both? Neither?

The argument starts here, and there are a variety of philosophical replies to the argument and to the thought experiment.

However, hearing the argument again got me wondering, is this argument scientifically sound? Is (1) really the case, and to what extent? Is (2) really possible? If so, is (3)? And how would a neurologist or psychologist answer (4)?

I’m curious. This is an area where I’ve read a bit of the philosophy but know virtually nothing about the science. I’m keen to learn what the people in this forum know about these things.


I don’t know about the former part, but the latter part of that sentence seems rather uncontroversial to me. Whatever role brain hemispheres play in self-identity, in so far as one brain hemisphere alone is able to sort of “run” a personality, that part of the brain is going to feel like it is still itself? Who else should it feel like being?

One problem I see with a premise like this is the unavoidable vagueness of terms like personality, self-identity, and what should constitute a “significant” measure of that. There doesn’t seem to be an objectively quantifiable measure of personality.


I’m very skeptical of (2) and of (3).

I’m not at all sure of (1), but I note that he uses “significantly” as a qualifier. And people might disagree on what they see as significant.

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I don’t think neuroscience rules out some sort of second substance soul. However it’s important not to neglect the role of embodiment in our sense of self-identity. The fact that the two brain halves are connect by sharing the same body may be sufficient on itself to give an integrated sense of self.


That makes sense. The question of how many substances or kinds of substances there are seems like a philosophical question, not a scientific question.

But science can have some broad and profound implications on philosophical questions. If this thought experiment is possible, then it seems to carry consequences for philosophy of mind, depending on one’s intuition about what is the outcome of the thought experiment. Say, Alice goes through the operation and her right and left hemispheres are placed in the brainless bodies of Betty and Carol respectively. The mutually exhaustive possible outcomes are:

  • Alice is (in) Betty’s body and Alice is (in) Carol’s body: Philosophers tend to find this option implausible because it doesn’t seem like one person can be in two places at the same time, and/or because each individual should have a single history, and/or because I can’t lack access to part of what it’s like to be me. Full disclosure: this is the option I tend to find most plausible.

  • Alice is not (in) Betty’s body and Alice is not (in) Carol’s Body: The operation kills Alice. This is a popular answer for many materialists, but it does have some challenging consequences. Does (1) above kill the patient? If yes, then there are some serious moral questions about ever performing the operation (1) and also some philosophical skepticism arises about how I know that I’m the same person over time. If no, then why is the existence of Alice dependent on what is done with the other half of her brain? If I destroy the other half, Alice survives, but if I place the other half in another body, Alice dies? What is I save Alice’s second hemisphere for 10 years and then move it to another body, will Alice cease to exist right at that moment? There are answers to these problems, but each of these answers involves some generally undesirable implications for a materialist philosophy of mind.

  • Either Alice is (in) Betty’s body or Alice is (in) Carol’s Body: In this case, the question becomes, which one? The determining fact(s) that answer this question are either facts about physical things, presumably the brain and bodies involved, or are facts about non-physical things.

    • If about physical things, the challenge is what physical fact bears on which body Alice ends up with? It seemingly can’t be which body is first for the operation. The operation could be performed at the same time, or close enough that who is first would depend on the observer’s reference frame. Maybe it depends on which hemisphere? Swinburne dismisses this based on appeal to neurology, and I don’t know the science well enough to know whether his appeal is valid. And it seems strange to think that something about Betty’s body or Carol’s body determines what body Alice ends up with.

    • If the explanation for which body is Alice’s is non-physical then it seems that whatever determines Alice’s self-identity, her being, is immaterial. It also seems to be simple, if it cannot be divided (and it seems it cannot be divided, else why reject the above possibility?). Alice seems to be a simple immaterial substance.

I don’t think the philosophical argument is even close to cut and dried (forgive the pun). But I think that accepting the scientific possibility of (1), (2) and (3) above raises the question (4), and since one seemingly reasonable answer to (4) entails substance dualism, the scientific possibility of this thought expeirment provides some evidence for substance dualism.

This is significantly more than simply saying that science rules out some sort of second substance soul. It seems to suggest science provides evidence for the existence of this soul, or, more precisely, strong evidence for a premise in an argument, the conclusion of which is that the immaterial soul exists.

I personally don’t think it does, for both philosophical and intuitive reasons. What I’m most curious about is whether this argument could be challenged scientifically as well. Is the possibility of this thought experiment settled in neurology, or is it debatable, controversial, or is it not likely possible? I’m curious.

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Have you excluded a middle ground? What if people can be partly in one place and partly in another?

Related, what if consciousness is divisible?

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We’ll have to wait until the time we can do this experiment, if we ever can (ethical obstacles may be more formidable than biological ones). Then we’ll have to ask Betty and Carol very carefully devised questions and listen very carefully to their answers. Otherwise, we just don;t know.

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That’s a good point. Most philosophers consider identity and existence to be all-or-nothing properties, but it may be that they allow values between zero and one. It may be that A and B are partially identical, or that A exists insofar as X. Baruch Spinoza and Michael Della Rocca both think identity and existence are properties of this kind (see Della Rocca 1996). I’m very sympathetic to this view, although I think it suffers from some apparent logical contradictions and so I don’t accept it myself.

If you do accept this view about identity or existence or both, then it seems I can be partly in one place and partly in another. If my self is divisible, so I could cut off a part of my self and, if I can destroy it, I would only partly exist*.

I think the options above are still mutually exhaustive (though not exclusive). Just add ‘(partly)’ after every ‘is’.

*This gets to an idea I had for a sci fi story which I don’t have time to write. It would be some future ‘Ship of Theseus’ surgury, where someone with a brain defect would have very small parts of their brain removed and replaced with special brain tissue that would map into the missing region and adapt itself there. Over a period of months, the entire brain would be replaced this way, and the story would leave unanswered the question of whether the original patient ceases to exist, and if so, at what point.

Della Rocca, M., 1996. Representation and the mind-body problem in Spinoza . Oxford University Press