The craniopagus twins from British Columbia: A test case for Thomistic dualism

The story of Krista and Tatiana Hogan, 11-year-old twin girls born in Vancouver, British Columbia, who are joined at the top, back, and sides of their heads, will doubtless be familiar to most readers. Here’s a documentary that was made about the twins:

Over at Uncommon Descent and Evolution News and Views, Denyse O’Leary, David Klinghoffer and Michael Egnor (who is a Professor of Neurosurgery at State University of New York, Stony Brook), have argued that the Hogan twins constitute a living refutation of the materialistic notion that mind and brain are one and the same thing. Each twin has her own distinct mind and personality, despite the fact that they share parts of their brains.

In this post, I’m going to argue that while these three authors are perfectly correct in insisting that each twin does indeed possess a mind of her own, their claim that this fact refutes materialism is profoundly mistaken. On the contrary, I will argue that the twins’ ability to share thoughts without speaking weakens the case for Thomistic dualism, and lends support to a subtle variety of materialism which incorporates top-down causation. And on this point, the mother of the Hogan twins would probably agree with me: she sends her children to a school run by the Seventh-day Adventist Church, which teaches that the human brain is the seat of thought, language, creativity, morality and even our consciousness of God. Could this Christian version of materialism be correct, after all?

For readers who are interested, here are a few articles on the Hogan twins:

Facts about the Twins by CBC “Doc Zone” (with Ann-Marie MacDonald)

Could Conjoined Twins Share a Mind? by Susan Dominus, in The New York Times Magazine, May 29, 2011. A well-researched and very thought-provoking article.

Inseparable. (See also this link here.) Shown on CBC-TV, November 5, 2017. This TV episode can be viewed only in Canada, but the accompanying article is worth reading.

Two minds that can share experiences and even thoughts

Let me begin by quoting an excerpt from Professor Egnor’s article about the Hogan twins:

The twins share portions of their brains and a common blood flow to their brains. They can never be safely surgically separated. The most important brain structure that they share is a thalamic bridge, which is a bundle of nerve axons. A similar connection exists in all people, and it connects the two sides of the thalamus. The thalamus is a critical part of the brain that mediates wakefulness and motor and sensory function.

Because the girls have a common thalamic bridge, they share control and sensation in several of their limbs. Krista has sensation and motor control over both of her own legs, and sensation and control of Tatiana‘s left leg. Tatiana has sensation and motor control over both of her own arms, and sensation and control of Krista‘s right arm. It appears that the common control over their arms and legs is restricted to automatic unconscious movements, not to deliberate planned movement.

In addition, the girls share some aspects of vision from each other’s eyes as well as taste. It also appears that the girls share some thoughts: they will laugh at things together without speaking, as if they have the same thought in their minds. (Emphases mine – VJT.)

The article accompanying the link to the CBC-TV program Inseparable, which was screened in Canada on November 5, 2017, confirms Professor Egnor’s observation that the two girls can share thoughts without talking, but points out that they remain “two distinct people”:

Neurological studies have stunned the doctors. Tatiana can see out of both of Krista’s eyes, while Krista can only see out of one of Tatiana’s. They also share the senses of touch and taste and the connection even extends to motor control. Tatiana controls 3 arms and a leg, while Krista controls 3 legs and an arm.
Amazingly, the girls say they also know one another’s thoughts without needing to speak. “We talk in our heads” is how they describe it.

Despite their unique connection, the twins remain two distinct people. Tatiana is talkative, outgoing and high-strung, while Krista is quieter, more relaxed and loves to joke. But she has a temper and can be aggressive if she doesn’t get her way. (Emphases mine – VJT.)

Writing in The New York Times Magazine (May 29, 2011), Susan Dominus is more cautious, suggesting that the girls are somehow both one and two individuals:

The girls surely have a complicated conception of what they mean by “me.” If one girl sees an object with her eyes and the other sees it via that thalamic link, are they having a shared experience? If the two girls are unique individuals, then each girl’s experience of that stimulus would inevitably be different; they would be having a parallel experience, but not one they experienced in some kind of commingling of consciousness. But do they think of themselves as one when they speak in unison, as they often do, if only in short phrases? When their voices joined together, I sometimes felt a shift — to me, they became one complicated being who happened to have two sets of vocal cords, no less plausible a concept than each of us having two eyes. Then, just as quickly, the girls’ distinct minds would make their respective presences felt: Tatiana smiled at me while her sister fixated on the television, or Krista alone responded with a “Yeah?” to the call of her name.

Although each girl often used “I” when she spoke, I never heard either say “we,” for all their collaboration. It was as if even they seemed confused by how to think of themselves, with the right language perhaps eluding them at this stage of development, under these unusual circumstances — or maybe not existing at all. “It’s like they are one and two people at the same time,” said Feinberg, the professor of psychiatry and neurology at Albert Einstein College of Medicine. What pronoun captures that?


As profound as it is to consider that each may witness the other’s consciousness, equally striking is their ability to maintain their individuality. In his book, “Altered Egos: How the Brain Creates the Self,” Feinberg describes patients with various split-brain syndromes, cases in which the corpus callosum, the part of the brain that serves as a bridge connecting one hemisphere to the other, is severed. In one manifestation, a patient might find that one of his hands is at odds, or all-out war, with the other. The unruly hand might throw a spoon or tear up money — actions that do not originate with any desire of which the patient is aware. Yet aside from the alien hand, the patient still feels essentially like himself: such patients “act, feel and experience themselves as intact,” Feinberg writes. Feinberg says the brain labors to create a unity of experience, knitting together our partial selves via numerous cortical mechanisms into a unified whole, into a sense of self, a consistent feeling of individuality and agency.

That the girls each have clear distinction, despite what he considers to be the likely leakage of sensory impressions, was telling to Feinberg. “With the split brain, you essentially cut the brain in half, yet the person feels and acts as a whole,” Feinberg said. “In these girls, they’re linked, yet each acts as a whole. It’s like a force of nature — the brain wants to unify.”

To the family, questions about whether the girls are two or one are so absurd as to be insulting. They are “two normal little girls who happen to go through life sharing a bubble,” [the girls’ mother Felicia] Simms said. (Emphases mine – VJT.)

Why I believe Professor Egnor’s refutation of materialism fails

For my part, the fact that the girls have distinct personalities, food preferences and even (on occasions) conflicting intentions constitutes sufficient evidence to show that they are indeed two distinct people, despite the fact that they share many connections between their brains. Does this disprove materialism? I hardly think so: after all, the girls have two brains, not one, even if those brains are uniquely inter-linked. I therefore find it difficult to understand why Denyse O’Leary, David Klinghoffer and Professor Egnor stoutly maintain that the case of the Hogan twins disproves the thesis that “Brain = Mind.” In any case, few materialists would equate the brain and the mind. It’s more common for them to claim that the mind is what the brain does.

In a recent podcast (ID the Future, July 7, 2017), Professor Egnor contended that even this more refined version of materialism cannot account for the reality of free will, which (as interviewer Casey Luskin pointed out) our legal system assumes that we possess:

I think it’s extremely difficult to make a coherent case that you have free will if you’re simply matter. That is, that matter is governed by the laws of physics, and free will plays no role in the laws of physics that we know of: there is no Newton’s law of free will. So if the human mind is the human brain, and the human brain is simply a physical object, it’s very difficult to ascribe either credit or blame for a human action… (9:40)

In a recent Skeptical Zone post (“Coyne’s latest defense of determinism: why it fails,” January 2, 2017), I argued that although the laws of physics make no reference to free will, neither do they rule it out, and that if we are prepared to allow for the possibility of top-down causation, there would be no need for determinism to be true, even if it should turn out that our choices are made by our brains (instead of being made, as Egnor believes, by our immortal and immaterial souls):

Even if we accept that thoughts and decisions are actions performed by our brains, and that these actions can be fully described in physical terms, the truth of determinism does not follow. To rule out the possibility of free will, we need to import an additional premise: that any movement of a body can always be fully explained in terms of the interactions between its physical constituents (i.e. the particles of which its is composed) and the other bodies in their surroundings. This premise might sound trivially obvious, but what it denies is that bodies have any holistic physical properties which affect the way in which their constituent particles move. When you put it that way, it’s not at all obvious.

Suppose now that brains possess holistic properties (which are physical but not attached to any particular part of the brain), and that thinking and choosing are simply holistic properties of conscious, normal human brains. Then since physics makes no reference to holistic properties in its mathematical descriptions of bodily motion, it follows that physics will be unable to fully account for the movement of any body possessing these holistic properties – which in turn means that physics can provide us with no guarantee that a set of inputs acting on the brain when its constituent particles are configured in physical state S will always generate the same output, since the brain’s holistic properties may turn out to vary, even when its constituent parts (i.e. its neurons) are configured in the same state.

In the same post, I sketched an account of how libertarian free will could still work, even if the brain (and not some immaterial faculty of the soul) makes choices. Choices, I argued, are holistic events in the brain, which constrain not only the spatial pattern but also the temporal pattern of neuronal firings in the brain, leaving them random at the quantum micro-level, but imposing a distinctive non-random pattern at the macro-level, which varies with the choice being made. In two follow-up comments (here and here), I went on to flesh out my account, and I argued that if a bodily activity were: (i) holistic, (ii) unpredictable and (iii) an embodiment of an act of will, then it would be a free choice, in the libertarian sense of “free.” Finally, I rebutted a common argument against materialism – namely, that to the extent that my brain influences my behavior, it does so by virtue of its physical properties rather than its volitional properties, which means that the kind of causation involved is non-rational. The fallacy lies in the assumption that causal properties cannot also be volitional. I maintained that there is nothing to prevent the non-local, holistic properties of (say) the prefrontal cortex from being both causal and volitional at the same time.

What is Thomistic dualism, anyway?

Professor Egnor is on the record as stating that his preferred philosophy of the relationship between mind and body is Thomistic dualism. Many readers will have heard of Cartesian dualism, the doctrine that each of us is two things – an immaterial mind plus a material body, which causally interact, in some mysterious fashion. Because Cartesian dualism views a human being as an amalgam of two things (soul plus body), it can be described as substance dualism. Additionally, the decisions made by the mind are held to be voluntary: they are not determined by what goes on in our bodies. Brains don’t hold thoughts, on the Cartesian view, so since the girls have separate minds, there’s no reason why the Hogan twins should be able to know each other’s decisions before publicly announcing them. And yet they do. As Susan Dominus writes in her article: “When the girls wanted to wash their hands in the sink, they worked as one, silently, to drag the bench over to the bathroom. More often than not, they both seemed to want to slither like snakes at the same moment, to roll a ball down a ramp to the television room, to drift toward the electric piano. But acceptance, rather than mutual desire, might be at play: the family often reminds them they have no choice but to compromise, and [the girls’ mother Felicia] Simms believes they have a private logic for determining whose turn it is to decide their whereabouts.”

Thomistic dualism, maintains that a human being is not two things, but a single entity (a person), who performs various kinds of actions. Some of these actions are bodily actions, and some are purely immaterial, non-bodily actions. Since Thomistic dualism views each human being as a single being, it would be wrong to characterize it as substance dualism; instead, it would be more fairly described as a form of action dualism, insofar as it ascribes both material and immaterial actions to human beings. Sensing, feeling, remembering and imagining fall into the category of material actions, while reasoning, understanding, loving someone and making a free choice fall into the category of immaterial actions. While we use our brains to store memories and to imagine things, we don’t reason, understand, choose or love with our brains. These are spiritual actions. Thomistic dualists readily acknowledge that when we think, we use our brains to call up images and memories from the past, but they insist that the actual thinking itself is not done in our brains. On a Thomistic dualist account, the very close connections between Krista’s brain and Tatiana’s brain could result in them being able to share each other’s sensations, feelings, images and memories, but not their actual thoughts or free choices. So when Krista formulates a syllogism (e.g. Mom can’t reach the ceiling, and I’m shorter than Mom, so I can’t reach it either), or grasps a new concept (say, the concept of an octagon), or virtuously resolves to take up a good habit (like exercising every morning), Tatiana wouldn’t automatically know the specific content of Krista’s thoughts or choices.

Why I think the Hogan twins’ ability to share thoughts poses a real difficulty for Thomistic dualism

In his recent article over at Evolution News and Views, Professor Egnor downplays the Hogan twins’ ability to read each other’s thoughts:

This sharing of some aspects of the mind, but not others, is remarkably consistent with classical Thomistic dualism. In Thomistic dualism, the human soul is the composite of three powers: vegetative, sensory, and rational. Vegetative powers are what we today call autonomic physiological control — control of heart rate, control of blood pressure, control of growth, reproduction, respiration, hormonal control, etc. These are unconscious powers that make life possible in the most fundamental way.

We also have “sensory” powers, which include (in modern terminology) sensation as well as motor function, and imagination and emotion. These vegetative and sensory powers of the mind are common to those of plants and animals, who have physiological (plants) and motor and sensory (animals) powers akin to those of humans.

Human beings alone have rational powers, which is the ability to think abstractly, without reference to particular things. Abstract mathematics, and abstract thought about logic or morality, are examples of rational powers of the human soul that are not shared by plants or animals.

Vegetative and sensory powers of the soul are material powers. They are tightly linked to brain function and are wholly the result of physical processes. However, rational powers of the soul — abstract thought — are immaterial powers

In light of this Thomistic understanding of the soul, the abilities that Tatiana and Krista share are the material powers of the brain, which we expect them to share, because they share brain matter. What they don’t appear to share is the immaterial aspect of the soul — reasoning in an abstract sense, and personal identity, individuality, etc. They are separate souls who share some material brain tissue, and thus share some material powers of the mind. They do not share immaterial powers of the mind, because immaterial powers can’t be shared, because immaterial powers aren’t material things that can be common to two people.

I find this argument unpersuasive. Professor Egnor himself admits that it “appears that the girls share some thoughts: they will laugh at things together without speaking, as if they have the same thought in their minds.” If Tatiana’s power to think abstract thoughts is an immaterial power, then why, I would ask, is Krista often aware of the content of these thoughts? And why are the girls able to “talk in their heads,” as they describe it? On a Thomistic account, there’s absolutely no reason why they should be able to do that.

In reply, Professor Egnor could argue that the girls are able to know each other’s thoughts by virtue of the fact that they “share some aspects of imagination — that is, the ability to reconstruct sensory images (visual, auditory, olfactory, etc.).” As he explains it: “In the Thomistic understanding, material powers ‘present’ information to the immaterial rational aspect of the soul, which abstracts the intelligible species (immaterial form) from the matter and comprehends it.” So even if the girls’ immaterial acts of reasoning and choosing are entirely separate, nevertheless, they may be able to know the content of each other’s thoughts by virtue of the mental images they share.

I find this proposal defective, on two counts. First, it would only work if our mental images determine the propositional content of our thoughts – which they don’t. For even though our reasoning makes use of mental images, which may be stored in the brain, it’s still possible for two people to make use of the same mental images for entirely different purposes: indeed, they may even use them to arrive at opposite conclusions. (For example: two people could have identical images of an unborn child in their heads, even though one is entertaining the thought, “Killing a fetus is right, if that’s what the mother wants,” while the other is entertaining the thought, “Killing a fetus is morally wrong.”) In other words, the content of the mental images which Krista and Tatiana share wouldn’t be sufficient to determine the propositional content of their abstract thoughts. So if they can actually read each other’s thoughts, a Thomistic dualist would be unable to explain this surprising ability.

Second, the girls’ own description of their remarkable ability to share their thoughts seems to indicate that they can indeed share propositional content: “We talk in our heads” is how they describe it. In other words, what they are sharing is not merely images, which might be called the “raw material” of thought, but the actual thoughts themselves.

A simple test that could decide if Thomistic dualism is true or false

I would like to propose the following simple test, which could decide once and for all if the Thomistic version of dualism is true or false. The test I had in mind would involve writing the following six sentences out on two sheets of paper, and showing one sheet to Tatiana (placing it in front of the eye Krista can’t see out of) and the other sheet to Krista (of course, Tatiana will see it too, but that’s OK). It’s vitally important that Krista and Tatiana don’t talk during the experiment. Of course, they are welcome to share their thoughts, by communicating mentally, because that’s the whole idea of the experiment: to see how much they can share.

Here are the six sentences:

1. Being healthy is better than being rich.
2. Angels are real.
3. Nobody should ever smoke.
4. One day, people will be able to go back in time.
5. Cheating is always wrong.
6. You can make any shape you want, using only triangles.

I believe that the two girls are now 11 years old. Since they’re still in elementary school, I’ve deliberately written the sentences in simple English, and I’ve kept them as short as possible, as I’d like the experiment to be a stress-free experience for the girls. At the same time, all of the sentences involve abstract concepts, such as “should,” “wrong,” “healthy,” “triangle,” “time” and “real,” so they involve higher-level thinking – the kind of thinking which both Cartesian and Thomistic dualists insist we can’t possibly do with our brains.

Here’s what each girl has to do. First, silently read the six sentences. (Alternatively, if either of the girls has a reading difficulty, the girls’ mother could read the sentences to them.) Second, pick one sentence which interests her (it doesn’t matter which), without saying which one it is. Third, mentally decide whether she agrees with it or disagrees with it. Fourth, in her mind (not out loud), say why she agrees with it. That shouldn’t take more than about 20 seconds.

Now here’s the test. Does each girl know (a) which sentence her sister picked, (b) whether her sister agrees with that sentence or disagrees with it, and (c) why her sister agrees or disagrees with it? If each girl (or even one girl) knows (a), (b) and (c), in relation to her twin sister, then I think that would rule out both Cartesian and Thomistic dualism. A Thomistic dualist would be able to explain (a), because Tatiana can see whatever Krista sees, so she’d probably notice which sentence Krista’s eye zoomed in on. A Thomistic dualist might even be able to explain (b), because it’s a binary decision – agree or disagree? – and when saying “Yes” or “No,” each girl may mentally conjure up an image of a Y or an N, or something like that. However, a Thomistic dualist would be unable to explain (c), because it involves abstract reasoning, which is not supposed to occur in the brain at all: it’s supposed to be a purely spiritual act. Even though such reasoning makes use of mental images stored in the brain (which the girls can share with one another), it’s still possible for two people to make use of the same mental images for entirely different purposes: indeed, as my example above on abortion shows, they may even use them to arrive at opposite conclusions. In other words, the content of the mental images which Krista and Tatiana share wouldn’t be sufficient to determine the content of their abstract thoughts. So if they can actually read each other’s thoughts, a Thomistic dualist would have no way to explain this surprising ability. But if it turns out that the girls are unable to read each other’s thoughts at this refined, abstract level, despite being able to share lower-level thoughts, then that would constitute powerful experimental evidence in favor of Thomistic dualism.

One last variant: the girls might want to repeat the experiment, but this time, trying as hard as they can not to share their innermost thoughts. It would be interesting to see if one of them managed to conceal her thoughts from the other. My guess is that they can do this, and that it may involve one of the girls shutting down the thalamic connection between her brain and her sister’s brain for a short period, by an act of will, but I could of course be very wrong about this.

I believe the Hogan sisters have a Facebook page. Unfortunately, I’m not on Facebook (for personal reasons), so I have no way of contacting the girls’ mother. If one of my readers would be kind enough to contact the girls’ mother and let her know about the experiment I have proposed in this post, I would be most grateful. Readers are also welcome to pass on my email address to the girls’ mother.

A concluding thought from the Seventh-day Adventists

As I mentioned above, Seventh-day Adventists espouse a form of materialism: they hold that the brain is the seat of both our higher and lower mental faculties. They consider the soul to be a spark of life breathed into the body by God; at death, the soul loses all consciousness, until the body is raised to life by God’s command, at the Resurrection.

In an article Brain and Mind: A Christian Perspective, by Linda Mei Lin Koh (Department of Education, Southeast Asia Union College, Republic of Singapore), which was prepared for the International Faith and Learning Seminar held at Union College, Lincoln, Nebraska, in June 1993, the author outlined her Seventh-day Adventist view of the human mind, and how it influences her outlook on life and her personal conduct:

How does the Christian view the brain or the mind?

Roger Sperry, one of the foremost exponents of split-brain studies, advocates a “unifying view of mind and brain” in the 1977 American Psychologist. According to Sperry, the mind is an emergent property of brain activity. Once the mind has emerged, it assumes the dominant role of driving the brain (Popper and Eccles, 1977).

In recent years, neurobiologists have produced research that enhances our understanding of the human mind. Fischbach in an article published in the 1990 Scientific American, identifies the brain as “the organ of the mind.” According to Fischbach, the brain, with its many specialized functions, is the central organ of the body. From the collective activity of all the brain regions emerges the most fascinating neurological phenomenon of all: the mind.

In agreement with Fischbach is Carla Shatz, professor of neurobiology at the University of California, Berkeley. She clearly asserts the fact that “the brain is the central organ that directs the intricate functions that make possible memory, vision, learning, thought, consciousness and other properties of the mind. . . In fact, during fetal development, the foundations of the mind are laid” (Shatz, 1990, p. 35)…

For the Seventh-day Adventist Christian teacher, the brain is viewed as more than an anatomical organ. It is a marvelous organ created by God (Genesis 1). It is a complex organ that directs and interprets our sensations, thinking, reactions, evaluations, and helps us to discriminate right from wrong, good from bad…

The brain is the structure that truly sets us apart from the rest of the animal kingdom. Together with the spinal cord, the brain forms the central nervous system that regulates our sensory, cognitive, emotional, physical, and motor abilities…

Several brain features have been identified. Among the general features of the brain are speech centers. The Broca’s area and the Wernicke’s area have been found to be responsible for the production of speech sounds as well as for the understanding of these sounds. Language is a significant means of communication with other individuals. It is essential for the conceptualization and elaboration of abstract ideas, the invention of ideas, and the understanding of the world around.

Through language, the human brain is capable of helping man look at himself as a person, thus developing self-knowledge. Self-knowledge, self-awareness, insures that human beings continuously seek to ask questions about themselves, their existence, their destiny, and everything about the world around.

Therefore, if the brain with such language capacities distinguishes the humanness of man and women, then implicit within this humanness is a potential for responding to the overtures of God. Individuals may respond warmly and enthusiastically, or just mildly, or even with outright hostility. Nevertheless, all our responses signify an interaction with God, something uniquely human.

Another magnificent function of the brain is the ability to create new ideas and solutions. According to cognitive psychologists, the higher part of the brain around the cerebral cortex seems to display some traces of creative ability…

Contrary to Skinnerian behavioristic belief that humans are controlled by the environment, D. Gareth reiterates that man is a thinking being capable of making value judgements, deciding on rightness or wrongness of ethical systems, and making moral decisions…

From a Christian perspective, the moral conscience of the human is vitally linked to the brain nerves that connect our mind with heaven. It is through this mental link that makes it possible to arouse moral concern (White, 1903).

Yes, the brain is the seat of thought and intelligence. It can evaluate, make critical judgments, analyze, generalize, and perform other functions. If voluntary taking of psychotropic drugs on a regular basis can modify behavior patterns tremendously, what responsibility do we have as Christians. Even as a human being, if perception is affected, thinking beclouded, judgment impaired, how can one function as a useful being at his optimal level?…

The human brain is complex and fragile. Its potential is still to be realized. The fragility of the human brain is a manifestation of human finiteness. Humans are limited because we are creatures in a God-ordered and God-sustained world. Wonderful as the brain is, it is part and parcel of our finiteness, as demonstrated in our vulnerable dependence on its integrity. There is much of which we are capable, but also much of which we are not capable. A Christian recognizes his dependence on God as well as the authority, responsibility and control bestowed on him by God. (Emphases mine – VJT.)

For me, this a very beautiful way of looking at the marvels of the brain, from a Christian perspective. And as far as I can tell, it’s a perspective which allows one to maintain belief in libertarian free will, which is fundamental to our legal system. Finally, I think it is a perspective which does justice to the shared experiences of the Hogan twins: two distinct and autonomous individuals whose brains are linked in a way that’s unlike anything physicians have ever seen before.

What do readers think? Is there anyone who would like to defend Thomistic dualism? If so, please feel free to comment. And now, over to you.

52 thoughts on “The craniopagus twins from British Columbia: A test case for Thomistic dualism

  1. walto,

    nope. Its conclusions from the bible. IOts very simple and clear because Gods word is meant to be understood and its boundaries and instruction. The bible instructs on humans identity. This because its just a mirror of God.

  2. Robert Byers: nope. Its conclusions from the bible

    Your own cuckoo version of it anyhow.

    Well, maybe that’s too harsh. So l’ll give you one last chance. What in the Bible suggests to you this mind-soul distinction (based on memories) that obviously tickles your fancy so much?
    Five or six passages will be enough for me. Either testament.

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