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Does Split-brain Surgery Show That We Have No Real Identity?

A prominent philosopher built his career arguing against the existence of personal identity

One of the most bizarre consequences of the modernist materialist conception of human beings is the notion that personal identity is not real or not continuous. This view is, of course, contrary in every way to the lived experience of each of us.

Like everyone else, I am the same person I was as a child, and the same person I will be a moment before my death. I am me, and I am no other. Of course, at different times of my life I have had different memories, experiences, and perceptions, but it is the same I (the only I) that has them. This is so fundamental to reality that it seems beyond question.

It is not even clear what personal identity would mean if it were not real and continuous. If my identity were not continuous, in what sense could I even think about it, given that “I” am a collective? Even posing a question about personal identity presupposes a unitary “me.” A bus full of passengers getting on and off does not contemplate personal identity—it does not contemplate at all.

The cause of this bizarre nonsense (the denial of personal identity and continuity)—aside from stupidity and even madness—is the modern materialist conception of man. Matter can be divided and lose its nature; it can be transformed into something else. Copper and silicon can become a computer, then a watch, then metal fragments, then metallic dust. Matter can be split, and if man is matter and nothing more, then he can be split. At that point, personal identity becomes problematic.

The astonishingly tenacious grip materialists hold on their ideological delusion is obvious from the fact that rather than deny materialism, they deny personal identity. Materialism must, in their view, never be doubted, even if it means doubting their own identity—even if it means doubting reality.

Derek Parfit (1942-2017, pictured in 2015) was a philosopher at Oxford, highly esteemed among his colleagues. He wrote extensively on the “question” of personal identity—he denied it, as you might have guessed. You may note the irony in being “a philosopher at Oxford who wrote extensively” and at the same time denying that you are “a” anything. One presumes that Parfit drew his salary from Oxford on a regular basis without confessing that each recipient was an impostor merely pretending to be the Derek Parfit who drew his salary last week. He no doubt enjoyed tenure, despite the fact that tenure presupposes individual identity and continuity (universities do not grant tenure to itinerant crowds).

Parfit felt liberated by the denial of his own identity:

My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness… [However] When I changed my view, the walls of my glass tunnel disappeared. I now live in the open air. There is still a difference between my life and the lives of other people. But the difference is less. Other people are closer. I am less concerned about the rest of my own life, and more concerned about the lives of others.

What could an identity-denier possibly mean by “my life… I was… I changed my view… my glass tunnel… I now live… my life… I am… my own life…” It’s odd for a crowd of incessantly shifting individuals to use such language. Outside a university classroom, the assertion that one is gratified by absence of personal identity would warrant psychiatric consultation, which is apparently an under-used resource at Oxford.

In 1971 Parfit published a paper in The Philosophical Review in which he explained the basis for doubting his personal identity. For the most part, it is, as you might imagine, a word salad only gently tethered to reality. He did, however, raise the interesting question of what happens to personal identity when the brain is surgically cut in half.

He misunderstands the answer to that question. I’m a neurosurgeon who has performed this operation and has treated such patients over many years, so I can answer it.

When a patient has had his cerebral hemispheres disconnected surgically (in order to treat epilepsy), nothing much happens. He is, by his own experience and in all normal activities of life, exactly the same as he was before his brain was cut in half. He is not in any sense “two people.” His identity is in no way altered or split.

Roger Sperry (1913–1994, pictured in 1981) was a neurophysiologist who understood the remarkable opportunity for neuroscience and philosophy of mind that patients with this surgery offered. He devoted much of his professional life to studying them. He too noted that in ordinary life they were completely normal.

By means of extensive and very clever experiments, he was able to discern perceptual differences after split-brain surgery. His results supported the standard theory that the right and left hemispheres of the brain subserve different perceptual and motor functions.

The right hemisphere subserves movement on the left half of the body, vision in the left half of the visual field, and spatial perception, among other things. The left hemisphere subserves movement on the right half of the body, vision in the right half of the visual field, and speech, among other things.

If an object is presented to a split-brain patient in his left visual field, he perceives it via his right hemisphere, and, lacking a connection to his left hemisphere (which was cut), he cannot name it, although he knows what it is (he can point to a picture of it). This research won Sperry a Nobel Prize for Physiology and Medicine in 1981.

Note that a patient whose brain has been split does not become two people. He is still one person, with perceptual disabilities like the inability to name something he sees via his non-linguistic (right) hemisphere. Likewise, a person who cannot speak or move his right side after a stroke hasn’t changed personal identity. He has a disability.

Sperry’s split-brain research did not break new ground in neuroscience because differential function of brain hemispheres was already well established from the study of strokes and brain injury. The localization of speech in the left hemisphere was known by the mid 19th century. Sperry’s work offered important evidence to confirm the prevailing view by studying an unusually specific, and therefore informative, kind of brain damage—surgical cutting of the connections between the hemispheres.

Sperry himself was no materialist. His philosophy of mind was “mentalist”:

By our current mind-brain theory, monism has to include subjective mental properties as causal realities. This is not the case with physicalism or materialism which are the understood antitheses of mentalism, and have traditionally excluded mental phenomena as causal constructs. In calling myself a ‘mentalist’, I hold subjective mental phenomena to be primary, causally potent realities as they are experienced subjectively, different from, more than, and not reducible to their physicochemical elements. At the same time, I define this position and the mind-brain theory on which it is based as monistic and see it as a major deterrent to dualism.

Bob Doyle, “Roger Sperry” at The Information Philosopher

Sperry, unlike Parfit and other materialists, understood that his research refuted materialism. Sperry also rightly rejected Cartesian dualism, which is the kind of dualism he refers to above. My own view of the soul-body relationship is Thomistic dualism, a more nuanced view which is consistent with Sperry’s mentalism.

The scandal is not that there are philosophers like Parfit who deny personal identity. There will always be frauds and madmen. And I mean to call them ‘frauds’—in every aspect of his life, Parfit lived as an individual with genuine persisting identity. How could he live as an itinerant crowd and continue to draw a salary, have relationships, teach, etc.? Men who live their lives in utter denial of that which they write and teach are nothing but frauds. The biggest scandal in our culture is that we take thinkers like Parfit seriously. We go to their lectures, publish their papers, and grant them tenure. And in doing so, we participate in fraud and madness.

Note: The photograph (cropped) of Derek Parfit at Harvard in 2015 is courtesy Anna Riedl (CC BY-SA 4.0)

You may also enjoy these articles by Michael Egnor on personal identity:

Interview with a woman (or women) formerly called Susan Blackmore: A professor of psychology argues that there is no continuity between our present selves and our past selves.

My right hemisphere is an atheist! No, wait… In reality, split-brain surgery does not split consciousness in any meaningful sense.

Yes, split brains are weird, but not the way you think. Scientists who dismiss consciousness and free will ignore the fact that the higher faculties of the mind cannot be split even by splitting the brain in half.


Some people think and speak with only half a brain. A new study sheds light on how they do it.


Four researchers whose work sheds light on the reality of the mind The brain can be cut in half, but the intellect and will cannot, says Michael Egnor. The intellect and will are metaphysically simple.

Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.

Does Split-brain Surgery Show That We Have No Real Identity?