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Science and Religion series -- "Science and the Soul"

Science and Religion series -- "Science and the Soul"

In this third essay in our Science and Religion series, Fr Jonathan Jong discusses an issue that comes directly from his own field, the cognitive and psychological sciences: questions of the soul.


There is a common idea—popular among Christians, but also many other people—that human beings are made up of two kinds of stuff, which we can call “body” and “soul”. People generally think that their bodies are physical, by which they mean something like that they are made of matter and obey the laws of physics. They are much less clear about what they think the soul is like, but the rough idea is that it’s not physical: sometimes, however, people imagine that souls—in the form of ghosts—can be seen or heard. It’s not obvious how this would work if they are not physical, given that sight and sound both involve the bouncing of waves off the surfaces of physical objects.

Philosophers often call this common view dualism [1]. I don’t have data on this to hand, but I think it is safe to say that most Christians are dualists. One reason for this is that it seems to be taught in the Bible. For example, there are words reasonably translated as “soul” (e.g., nefesh, psyche), “spirit” (e.g., ruach, pneuma), and “mind” (e.g., nous), as well as words that refer to the body (e.g., soma) or flesh (e.g., sarx). However, the meanings and uses of these words are varied and uncertain (e.g., both the Hebrew nefesh and ruach are sometimes rendered “mind”).

Another reason for the popularity of dualism is the widespread idea that we need to have a soul to also enjoy some other capacities—like free will and life after death—that are important to Christians. We’ll come back to these concerns later on.

Now, in contrast to many Christians—and not just Christians, but also many many other people from different religious backgrounds and none—almost all psychologists and neuroscientists reject dualism. Instead, they hold a view that is often called physicalism (or materialism), which means pretty much what it sounds like. This is a somewhat awkward state of affairs, seeing as the etymology of the word “psychology”—from the Greek psyche and logia—is plausibly rendered as “science of the soul”: as I briefly mentioned earlier, psyche is the word in the New Testament often translated as “soul”.

Nobody now really every talks about psychology as the science of the soul, and perhaps this is apt, as our modern conception of psychology is much narrower than those of our ancestors. For example, Aristotle’s starting point was that the soul is “the principle of animal life” [2], whereas modern psychologists are mainly interested in the mental aspects of that life: perception, sensation, cognition, and emotion. Partly for this reason psychologists (and philosophers) these days are more comfortable thinking of themselves as studying the mind.

Another way in which modern psychology departs from Aristotle is the emphasis we now put on the brain as the seat of mental life: especially in his On Parts of Animals, Aristotle locates the centre of sensation at the heart, with the brain mostly fulfilling a temperature regulation function. This is cause for some embarrassment among Aristotelians. In any case, as technologies for observing brain activity have improved—with the advent of EEG machines, PET scans, and fMRI—psychological science has increasingly involved neuroscience.

Approximately all psychologists and neuroscientists would agree that the mind is dependent on the brain. In other words, our mental activity—our thoughts and feelings, our memories, our experiencing the world, our deciding to act in it—require brain activity. Of this, we are sure, but of little else. We do not, for example, know exactly how specific thoughts correspond to activity in specific brain region: indeed, we are very very far from knowing such a thing, in part because it’s not clear how to distinguish one specific thought from another, so as to check whether they activate different brain regions in different ways.

I want to avoid both over-playing or under-playing the depths of our ignorance. We might not know how specific thoughts correspond to specific brain activity, but we do know a lot about how certain kinds of experiences and actions co-occur with specific brain activity. We know, for example, that there are neurons—brain cells—that fire when we see lines but not dots, and others that fire when we see dots but not lines.

Actually, we know rather a lot about the brain activity related to visual perception. We know that things on the left hand side of our visual field activate neurons on the right side of our brains, and vice versa: there are a few different phenomena, including movement [3], that are “crossed” in this way. We also know that vision involves almost every broad region of the brain: after light waves pass through the lenses of our eyes and hit the cells in our retinas, cascades of information (carried by electrical and chemical signals) trigger activity in the lateral geniculate nucleus at the centre of the brain, the occipital cortex at the back, and then the parietal cortex on top and the temporal cortex on the lower side.

Through careful experiments, we now know that these regions specialise in different aspects of visual perception, like the detection of edges, spatial organisation, colour, and even something we sometimes call whatness, which is the recognition of an object as that object. We know all these things in part from the brain imagining technologies I mentioned above, but another incomparably valuable source of information for us is patients—human and non-human both—with specific brain damage.

 Bennett et al.  The dead salmon study.  [4]

Bennett et al. The dead salmon study. [4]

So, there is a sense in which we know a lot about how different parts of the brain correspond to different aspects of our experience. But there are also many mysteries left to uncover, some of them emerging from new discoveries. For example, among the things we have learned from patients with brain damage and abnormality is that we enjoy a lot of neural plasticity, which means that the relationship between the brain and mental capacities is flexible, malleable. When one brain region is damaged, other parts of the brain can—albeit not fully—compensate for the damage: some individuals even live remarkably functional, if not quite normal, lives with vanishingly little cortical brain matter. This challenges the idea that we know how mental activity corresponds to brain activity, because it turns out that the relationships between the two might not be fixed.

We should also be careful when we are drawing conclusions about the nature and existence of the mind when we interpret neuroscientific research. As we have seen so far, the kind of evidence neuroscientists handle are correlational (e.g., from neuroimaging studies) and clinical (e.g., from patients or experiments with non-human animals [5]). To recap, what the evidence tells us is that mental activity always comes with some brain activity and that mental deficits arise when there is brain damage. In other words, the scientific evidence tells us that mental states and brain states are correlated and that brain activity is necessary for mental activity: it does not tell us that brain activity is sufficient for mental activity. This rarely stops neuroscientists from drawing that conclusion, but it should be noted that it is an unwarranted one from the scientific evidence alone.

Which brings us back to dualism. Almost all psychologists and neuroscientists reject dualism, in large part because they believe that brain activity is sufficient for mental activity. Put another way, they believe that neuroscience—sans the soul—is able to adequately explain mental activity, thereby rendering the soul redundant. In the history of science, this is a perfectly respectable way to stop believing in things, by showing that they are unnecessary.

For example, there used to be a puzzle about what happens when things burn. One dominant theory until the 1770s was that when things burn, a substance—phlogiston—is released. This theory faced problems when scientists discovered that things often got heavier when they were burnt, which is inconsistent with the idea that something is lost during combustion. Some people speculated that phlogiston has negative mass, which explained why (say) metals became heavier when they lost phlogiston. This was a clever idea; too clever by half. Eventually, scientists worked out that oxygen was required for combustion, and that it was the binding of oxygen to the metal that made them heavier. So, we no longer needed phlogiston to explain the change in mass: without any other independent reason to believe in phlogiston, we abandoned it [6].

[Humour me another example, this time from astronomy. In the 1840s, scientists observed something unexpected about the way the planet Mercury revolved around the Sun. As you might know, planetary orbits are elliptical rather than perfectly circular, and the Sun is not really at the exact centre of the orbit: this entails that there is a point of the orbit at which the planet is closest to the Sun as well as a point at which it is furthest. The former point is called the perihelion. From orbit to orbit, the perihelion shifts: this is called a perihelion precession. The details of a planet’s perihelion precession are predictable: specifically, they should be predictable based on Newton’s laws of motion and gravitation. In the 1840s, scientists tried to match observations of Mercury’s perihelion to the theoretical predictions, and found that the theory failed to predict the data. This was a big deal, because Newton’s theory was a very successful one in lots of different ways, and really the only viable candidate for accounting for many other observations. Instead of abandoning the theory, scientists supposed that there was a small planet located between Mercury and the Sun, which could account for the difference between the data and the theory. There were a bunch of attempts to find this planet, which they called Vulcan: none of these attempts were successful. Then, in 1915, Einstein came along with his theory of relativity that understood gravity differently from Newton: it turned out that Einstein’s theory predicted Mercury’s perihelion precession perfectly without the need to suppose that there is a hidden planet called Vulcan. So, we abandoned the idea.]

Psychologists and neuroscientists who reject dualism basically believe that the soul is like phlogiston (and Vulcan). And they might be right. But they have not discovered an equivalent to Lavoisier’s oxygen theory of combustion or Einstein’s theory of relativity. It is not even clear whether the current state of neuroscientific research—which is in the business of discovering the neural correlates and necessary conditions of psychological functioning—is best described as explaining human experiences and actions. More specifically, even as we accumulate neuroimagining data, debates continue over the neuroscientific tractability of phenomena like conscious experience and intentional action. Admittedly, these debates mostly occur mostly among philosophers: psychologists and neuroscientists just have faith that we will work it out in the end. [7]

There is, it seems to me, no neuroscientific argument per se against dualism. Now, this is not to say that there is no argument against dualism at all, just that there is no such argument from neuroscience. Nor am I saying that we should all be dualists: the lack of counterarguments is too low a bar for belief. If the physicalism of psychologists and neuroscientists is generally uncritically held, so is the dualism of most people, including most Christians. I began this essay with the observation that most people’s ideas about the soul are inchoate and inconsistent, but that they nevertheless feel that belief in the soul is important.

It’s true that the Christian theological tradition has always found much richness in the idea of the soul. However, throughout the ages, Christian theologians and philosophers have held a variety of views on how the soul/spirit/mind is related to the body. Some have been dualists in the sense I have been describing, but the most influential view among theologians and philosophers—which is to say nothing of Christians more broadly—is probably a view, inspired by Aristotle and expressed most clearly by Thomas Aquinas, called hylomorphism.

Hylomorphism is not primarily a philosophical theory about the soul: it is a theory about objects in general. It is the theory that (almost [8]) all objects—cakes, tables, people, societies—are compounds of matter and form. This should not be understood as meaning that things are made of two kinds of stuff: it is not a version of dualism (as it is now understood) per se, as the form of an object is not a part of that object in the way that flour and eggs and butter are ingredients in a cake. Objects are made of matter; the form is what makes a lump of matter the object that it is, like a statue or a cake, a Velociraptor or you. In the case of animals like dinosaurs and human beings, the form is called the soul, the principle of animal life.

Another way to think about Hylomorphism is to use another set of Aristotelian concepts, potentiality and actuality. Matter is potentially a particular object, and what makes it actually that particular object is its form. A lump of matter—think of a little pile of atoms, if you like, even though that doesn’t really make such physical sense—has the potential of being any number of objects; what makes it actually a cake or a Velociraptor is its form.

In recognising that there exists more than just material substance, hylomorphism avoids the kind of physicalism (or materialism) I have accused psychologists and neuroscientists of uncritically accepting. However, it is important to note that the primary function of the soul here is not to explain particular mental phenomena: it might contribute to explanations of phenomena like consciousness and intentionality, but it might not. What it does—according to Aquinas, and Aristotle before him—is make a lump of matter a particular individual object, and also a member of a kind. My soul makes me me, a human being. As the principle of animal life, my soul is also what distinguishes me from a lump of matter that looks exactly like me, but deader.

Now, as it happens, Aquinas did think that the soul explains some mental functioning, chiefly understanding and willing. The arguments for this position go beyond the scope of this essay, but they are motivated by Aquinas’s view that human beings are essentially rational animals: what is means to be a human being is just to be a rational animal. If, as above, the job of the human soul—as the form of the human body—is to make it what is, namely a human body, then the soul must be what supplies the rationality to that body. We might disagree with Aquinas on this, but even if we did, it would not pose a problem for the hylomorphic theory of the soul more generally.

Ignoring the previous paragraph, a physicalist might say: Wait a second, if the soul is just the form of the body—and not a separate kind of stuff—surely I can believe in it too. I could, for example, understand the soul as the way a body is arranged such that it is a particular object or organism. Furthermore, I obviously believe that there is a difference between living and dead bodies: I could understand the soul as the way the body behaves, which makes it a living body.  I think this is basically right. Indeed, there are many possible metaphors for a physicalist to make sense of the soul without committing to more than one kind of stuff. The soul might be structure or information. Take your pick.

Before we get too friendly with physicalists, there are things that people—including Christians—want to say about the soul that physicalists might not accept. When most people talk about the soul, they are not thinking about it as that which turns potential actual. What they want is something that is the real them, and that gives them free will, and which guarantees the possibility of immortality: when they die, it is their soul that they imagine goes to heaven (should they be so lucky blessed).

As Aquinas understood it, the soul isn’t your real you. You are essentially the compound of your soul and your body: without your body, you don’t really exist. However, the soul is what makes any given lump of matter you: so, it can serve as that which preserves your personal identity, even as the matter you are made of changes (which it does throughout your life [9]). And when you die, you no longer have your body, and therefore cease to exist. You might say that you continue to exist because your soul exists, but this is not quite right. In any case, if there is to be hope of your resurrection—as opposed to the resurrection of something that looks like you, but isn’t really—there had better be something that preserves your you-ness between your current life and your life hereafter.

When it comes to free will, Aquinas has an understanding of human agency that doesn’t sit neatly with how we tend to think of it now. One of the motivations for philosophers and theologians these days to believe in an immaterial soul is the worry that materialism makes free will impossible because matter behaves according to deterministic physical laws, and determinism is antithetical to free will. The idea that determinism is antithetical to free will is questionable; it is a subject of great debate, which I shall not summarise here [10]. It’ll suffice to say that it’s not obvious that determinism threatens human freedom, and so it’s not obvious that a soul is needed to guarantee said freedom. Aquinas did not seem to worry very much about this either, but he does have a view about the relationship between God and human action that people after him have worried about. Aquinas thinks that God is the cause of all things, including human actions: to people now, this threatens our freedom. But not to Aquinas, who believes that the possibility of human freedom—whatever you take that to mean—depends on God as the first cause, to whom freedom belongs properly: to think otherwise is to illegitimately conflate divine causation with finite causation.

It’s probably time to wrap up. I have tried to argue that neuroscience has not yet done away with the soul, even as substance dualists understand it. The soul is to psychological science as dark energy is to astrophysics: it is the name we give to whatever accounts for that which we do not yet understand—the observed acceleration of the universe in the case of dark energy, and first-person phenomena like consciousness and intentionality in the case of the soul. Psychological phenomena are much much less well-specified than physical phenomena: in this sense, the soul’s status as a hypothetical entity in a scientific theory is much poorer than that of dark energy. But psychology ain’t physics.

I have also tried to argue that Christians don’t have to be substance dualists anyway, and that the most important idea on this matter in the history of Christian through is Aquinas’s hylomorphic theory, the basics of which most psychologists and neuroscientists should be able to accept. But what they might not be able to accept—for reasons I shall leave them to provide—is the role Christians would argue the soul plays in the preservation of personal identity, so as to make the resurrection of particular individual persons possible. In other words, Christians do not so much believe in the immortality of the soul as the resurrection of the body, but the soul is crucial if the body that is raised is indeed your body.


  1. The term dualism is often qualified for specificity. When the stuff in question is a “substance”, the position is called substance dualism, and when it is a “property” then it is quite appropriately is called property dualism. For the most of the first half of this essay, I am talking about substance dualism.

  2. De anima, Book I, Pt 1.

  3. We know a lot about motion too: for example, the sensorimotor cortex (located toward the back of the frontal cortex) has a homuncular arrangement which Wilder Penfield depicted to great effect.

  4. This image is taken from a research paper that reported an fMRI scan of a dead salmon, which—surprisingly—found evidence of brain activation. It is not a demonstration of brain activity among the dead, but of errors that can creep into fMRI research. Clicking the image will take you to the paper.

  5. There are moral worries about these, but we won’t get into them here.

  6. This story is not quite right. Many chemists believed in phlogiston for a little while after the 1770s, saying more and more complicated things about it, as if negative mass was not bad enough. I say this, but quantum mechanics does allow for negative mass. As a general rule, readers should stop reading as soon as non-physicists start talking about quantum mechanics, so that is all I will say about the matter. No pun intended.

  7. There is an excellent paper by a philosopher, William Lycan—called “Giving Dualism its Due”—making the point that there are no successful arguments for physicalism and no successful refutations of dualism. The remarkable thing about this paper is that Lycan is himself a physicalist.

  8. Angels and God are, according to Thomas Aquinas, form without matter.

  9. This may not be quite true in detail. The general idea that the atoms in our bodies are constantly changing—as we breath and eat and drink new atoms, and expel old ones in a variety of ways—is correct. However, some of our bodily structures may not be affected by this constant renewal. For example, there is some evidence that adult teeth remain more or less identical in composition over time. There is now some evidence for material renewal in our brain cells, but not enough for me to be confident about it.

  10. Some philosophers also argue—rightly, I think—that it’s not really free will that we want, but a sense of moral responsibility. That shifts the question to being about whether determinism threatens moral responsibility, which I will also not get into: on this there is also a large literature.

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