Splitting the Brain Creates Two Minds?

A philosopher friend of mine just had her book reviewed. The questions here about split brains, the mind, and the soul touches on questions of AI and some recent exchanges here. What do you think? If we divide the brain, do we have two minds?

Though the literature is now large and sophisticated, Elizabeth Schechter’s is the first book-length treatment of the problem by a philosopher (though Marks 1981 is a short monograph published in book form). Schechter rethinks the issues carefully from the ground up, and makes connections to a range of other questions in the philosophy of mind. She defends a “two subjects” view: within the body of a split-brain patient, in at least some cases, two experiencing subjects exist. She combines this with the claim that a split-brain patient is just one person ; those two units need not line up in a simple way. In this review, I will concentrate on the two-subjects claims, Schechter’s treatment of what has traditionally been seen as the main question in this area.

Schechter has written an exceptionally good book. Its particular virtues are the care and rigor with which arguments are developed. The book seems to lack a single superficial moment. It also feels rather long and dense. Schechter makes use of a considerable amount of neurobiological detail, not all of which is made accessible to the non-expert – this is as much a cognitive science book as a philosophy book. Though it is not especially easy once one arrives at the central chapters, the book remains more readable than it might have been, because Schechter’s writing is very appealing – deft and stylish, in an understated way. This may help keep you going as you make your way through a lot of argument. The book seems to me an exemplary piece of naturalistic philosophy of mind.

Curious @Faizal_Ali’s thoughts on this…

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I don’t think there is any fact of the matter, here. What we mean by “mind” is not at all well defined.


Two (possibly) useful comparisons come to mind:

If we cut a banana in two, do we have two banana’s?

  • I would say no in this case. It would just seem that we have two pieces of a single banana, that are split from each other.

If we cut a piece of string in two, do we have two pieces of string?

  • I would say yes in this case

I’m inclined to think of the mind in the same way as the banana. However, these questions might not get at the real issue, which I would suggest is whether or not split-brains result in there being two persons.

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Once we agree that there is a subconscious mind (in addition to a conscious one), then it is unavoidable that we can have MORE THAN ONE subconscious minds!

But here is the thing: there will always be just ONE conscious mind… attempting to regulate or suppress the impulses of the subconscious minds!

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It looks like an interesting book.

I’m not sure if I am understanding the concept of “partial unity” correctly, but it seems to reflect my take on the split brain situation: I don’t think these patients necessarily present a difference in terms of organization into one vs two minds, so much as the disruption caused by the surgery allows us a window to better understand how the mind works under normal circumstances.

I think it could be argued that we do not have just one or two minds, but innumerable minds working constantly in our brain, processing information at multiple levels and from multiple sources, which is then filtered, integrated and presented to our conscious selves as a coherent experience. In split brain patients, there is a sudden physical insult that interrupts the tracts thru which this process usually occurs, so the integration is no longer as smooth, and dysjunctions can be demonstrated by controlled experiments. However, the subjective experience remains of a single unified mind because the areas of the brain responsible for creating this impression have not been affected.

My 2 cents.



Can you explain any examples or experience you have where a split brain might become evident in clinical testing or everyday behaivior? It seems some concrete stories and examples would be helpful.

Perhaps as interesting as split brains, is the case of siamese twins conjoined at the brain.

Here’s one of the well-known split brain subjects, “Joe”, showing some of the effects. You can skip the neuroanatomy stuff that you know and fast forward to a minute in:


Here’s some more examples from Gazzaniga, starting at 26:00.


The question must be reduced to make sense of it:

How many CONSCIOUS minds does a person have?

The cases of multiple personalities seem to indicate that there can be a risk-taking personality in the same conscious mind as a more cautious personality… but, how do we align speach with multiple personalities. In theater, they sometimes argue with dominance over overt behavior, over oral behavior… but is it really two different SOULS? Or is it one soul, switching back and forth regarding modality, or goal-setting?

If you want to reduce the issue to make it easier to discuss, I would suggest removing any discussion of “souls.”


Do you have a word that represents the uniqueness of the human person, that lies on “woo woo” spectrum - -

to the right of the word “personality”, but to the left of the word “soul”?

I’m not completely sure those two terms are even on the same spectrum.

However, I think a good term for what we seem to be discussing might be “self”.



That seems like a good bias-free term!: “Self”.

So, I would propose that no matter how many sub-conscious “modules” we might find in an average man or woman, if they all report to the same CONSCIOUS mind, then they are all part of “Self”.

The question remains as to whether “multiple personalities” represent more than one “self” or not.

I would suggest NO. What multiple personalities might mean is that one or more usually subconscious modules have become powerful enough to intrude into the CONSCIOUS field of the “self”.

I know I’m a psychiatrist so I should probably understand this better, but I’m not really up on the latest research on Dissociative Identity Disorder, as the condition is now named. But I think an alternative explanation to the one you suggest is that DID represents a variation in functioning of the integrative “interpreter” function of the left hemisphere that Gazanniga describes.


Ian McGilchrist, an English neuroscientist, wrote a book on the two hemispheres that deserves more attention.

The Master and his Emissary

For the TL:DRs there’s a TED talk:

The Divided Brain.

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Marks! My goodness. I was a student of his at University of Washington back in the 1970s. He spent a few weeks of one term talking about this split-brain stuff. It seemed to me that the question he asked, of whether there was one person or two (he didn’t speak of one “mind” or two, though I’m not sure that would make much difference), was not a great question. But, along the lines of Faizal Ali’s remarks here, I now think that he was sort of missing out on one of the most exciting aspects of it: that we were at a point in time when neuroscience was beginning to illuminate areas which had been, due to the strange history of the discipline of psychology, relatively dark.

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There is only ONE conscious mind.

There could be dozens of non-conscious neural systems vying for attention, control and/or attention.

The non-conscious systems leave ripples on the conscious field like a large mouth bass leaves ripples on the surface of a lake!

You’ll never grasp this from a first-person point of view.

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I hasten to add, neither will I; there’s a fundamental law of the Universe that any sentient organism cannot understand anything as complex as itself. :slight_smile: