Writing at The Skeptical Zone, V.J. Torley has a very thoughtful post about the mental abilities of the craniopagus Hogan twins. He takes issue with my view that, based on what (little) we know about the twins’ mental abilities, their mental powers are quite consistent with Thomistic dualism.

Torley writes:

In this post, I’m going to argue that while these three authors [O’Leary, Klinghoffer, and Egnor] 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.

He characterizes the Thomist perspective:

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.

There is some debate about the immaterial/material nature of memory. Certainly, memory of perceptions, images, and other material actions is inherently material. However, memory of abstract concepts may not be material, and much of what we call memory is a composite of material and immaterial powers. For example, my memory of calculus entails my recollection of images of equations (material) and my contemplation of the concept of the limit of infinitesimals, which is an immaterial abstraction. Some philosophers with sympathy for the Thomistic perspective argue that “memory” of abstract concepts, unlike memory of sensations or images, isn’t memory per se, but is actually retained knowledge. I remember equations, but I know the concept of limits. It’s a subtle point, and eight centuries after St. Thomas it’s still controversial.

Where Torley disagrees with my view that the twins’ mental abilities are consistent with Thomistic dualism is on their ability to share thoughts. I assert that what they share are images, which are material mental things on the Thomistic view, but that they do not share abstract thought, which is immaterial. (By “images” I mean reconstruction of sensations — visual, auditory, tactile, etc. — in the absence of the object originally sensed.)

Torley writes:

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.

To understand the Thomistic view better, it is helpful to consider how perceptual (material) and intellectual (immaterial) thought arise. Perceptual thought, which is material, arises when our sense organs grasp the form of the object sensed. For example, if we see a man working in a field, our perception of him working is the result of the grasping of the form (the visual image) of the working man. This aspect of form is called the “sensible species.”

Once the sensible species is grasped by the soul, the intellect, which is immaterial, strips it of particulars (which are material) and grasps the universal concept inherent to the man working in the field. The aspect of form grasped by the intellect is called the “intelligible species.” This extraction of the intelligible species from the sensible species is carried out by the active intellect, and is presented to the passive intellect, which is that power by which we contemplate (in an abstract sense) the man working in the field. We may contemplate how difficult his labor is, or how it reminds us of slavery, or of the plight of migrant workers, etc.

You will note that the material and immaterial powers of the mind work in tandem. All immaterial thought is ultimately derived from material (particular) sensations and perceptions, in the Thomistic view. The material mind presents forms to the immaterial mind, which extracts the universal abstractions from the particular sensations.

It is clear that the Hogan twins share some thoughts — they giggle at private thoughts that they seem to share. Torley believes that these private thoughts entail some immaterial content. I believe they do not. I believe that the twins share images and perceptions, and that their reaction to the images that they share (giggling) is a manifestation that they both quite separately find the images funny.

In my view, the twins don’t share the intellectual immaterial thoughts. They do share some imaginary material thoughts (sensible species), from which they (at times) each individually extract similar immaterial thoughts (intelligible species). It is only the material sensible species that they share by virtue of their brain connection. They may both extract similar immaterial intelligible species from the shared material thought, but the intelligible species is extracted separately by each girl and is not shared, even though both girls have similar abstract thoughts about the particular perception or image that they share. That would be the view most consistent with Thomistic dualism.

Torley continues:

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.

I don’t see the evidence that they share propositions. “Talking in their heads” can mean many things, and may just refer to shared perceptions from which they independently derive similar propositions. They may arrive at similar propositions from their shared image, in the same way that two different people may look at the same object and draw similar conclusions from it.

Torley proposes a clever experiment to test whether the girls share merely particular sensible species or universal intelligible species as well.

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.

It’s a clever and interesting experiment, and I certainly think that these twins provide a remarkable opportunity to study the mind and to sort out issues of materialism and Cartesian and Thomistic dualism. But there are pitfalls in Torley’s methodology.

If the girls share intelligible species (immaterial thought) as well as sensible species (material thought), then each twin should know (a), (b), and (c). But even if they don’t share intelligible species, which is the Thomist view, they may still share sensible species/images and may separately derive the same intelligible species from it. This is particularly likely because of the close relationship between the two girls. They may routinely and individually derive the same abstract thoughts from a sensory image, yet not actually share the same abstract thought. This kind of thing is quite common, for example, with married couples over many years, who both think of abstract things at about the same time (my wife and I do this all the time). Similar things happen with non-conjoined twins and even with close siblings, for whom the actual sharing of thoughts is not an issue.

Because the immaterial intellect extracts universals from material sensations, the process of sensation, perception, and intellection are closely linked and generally simultaneous. It can be very hard to tease these acts out empirically, especially in conjoined twins who are likely to have developed a remarkable degree of rapport and similar experiences since birth.

I suggest another approach to testing the girls. Mathematics is a particularly clear composite of material perceptions and immaterial abstractions. We perceive written numbers and equations, and we think abstractly about the mathematical concepts they represent. We perceive and reason simultaneously in mathematics, but the perception and the reasoning are distinct powers — perception is material, and reasoning is immaterial.

If the twins share both material and immaterial powers of the mind, they should share mathematical ability very closely. They should have the same aptitude and the same comprehension of mathematical concepts at every stage of their education. If they share material and immaterial thought, they should share mathematical aptitude, and do so identically.

If they share perceptions (images), but not abstraction (concepts), they would be expected to differ at times in their mathematical aptitude. For example, both girls may share the perception for the symbol for a square root, but if they do not share immaterial thought, it is quite likely that one girl will understand what square roots mean before the other girl understands it. Now it is quite possible that the twins, even if they do not share abstract thought, may learn some or even most aspects of mathematics at about the same rate, given that they are always in the same environment and actually share sensations and perceptions. But if they do not share abstract thought, it is likely that in at least a few aspects of their mathematical education they will progress at different rates, because they have different comprehensions of the mathematical perceptions they share.

This can be tested rather easily. Thomistic dualism predicts that they will have at least occasional disparities in their understanding of mathematics, which would show up on standardized tests, school grades, etc. If they share intellect as well as perception, their scores and grades should be indistinguishable.

I would suggest that the mathematical test is more comprehensive and practical than the test suggested by Torley. If Thomists are right, the girls will diverge at times in their mathematical aptitude. If Thomists are wrong, and the girls share intellect and well as perception, they should not diverge at all.

I do point out, however, that in the Thomist view it is not merely empirically true that perception is material and intellection is immaterial. It is logically necessary for the intellect to be immaterial, because the intellect is that by which we contemplate universals, which by definition do not have particular existence and thus cannot be material.

If the twins were shown to share intellect as well as perception, it is not merely our theory of mind that would need revamping, but the logical and metaphysical basis for Western thought as well.

The Hogan twins provide a remarkable opportunity to deepen our understanding of the human mind, and to put our metaphysical and logical predicates to an empirical test. One test, which is the assessment of their mathematical abilities as they progress through school, is quite practical and easy to perform.

Photo: Krista and Tatiana Hogan, via YouTube.