Can Mind/Brain Supervenience be Demonstrated?

Can science demonstrate the relationship between the mind and brain?

Posted Oct 01, 2018

In C. S. Lewis’ Christian Apologetics: Pro and Con, edited by Gregory Bassham (2015), Victor Reppert and I had it out about (i.e., we collegiately debated) C.S. Lewis’ famous “argument from reason”—an argument which claims that naturalism (the notion that the supernatural does not exist) is self-defeating. The basic idea is that naturalism cannot account for the causal efficacy of mental operations like reasoning, which lead the naturalist to the conclusion that naturalism is true in the first place. I argued that the argument doesn’t work because, simply put, mental operations are not necessarily supernatural.

Since that debate, Reppert has been working on an additional reply to my arguments which will soon be published in the journal Philosophia Christi. For the past month or so, per the request of the journal’s editor, I have been working on a response. Now, I’m going to save the details of the debate for the book and the journal (when the latter is published, I’ll post a link here); but an issue arose during the debate about the relationship between the brain and mind which is not actually relevant to the debate, but nonetheless interesting. Because it would have taken my argument too far off track (and taken up too much space), and because the topic of the relationship between the brain and mind is much more appropriate for Psychology Today, I decided to move my argument on this topic here.

Defining the issue

What’s the issue? Whether the mind supervenes on the brain—or, to be more specific, whether the supervenient relationship between the mind and brain can be demonstrated scientifically. But to understand the issue, we must first understand what supervenience is. To explain it, let me quote my forthcoming reply:

“A supervenient relationship holds between two things when a change [i.e., difference] in one is not possible without a change in the other. X supervenes on Y if no change is possible in X without a change in Y. In my original argument, I used the example of a photo-mosaic that hung in my college dorm room which consisted of individual frames of the Star Wars Trilogy that formed a picture of Darth Vader. Vader supervenes on the frames; no change in Vader is possible without changing the frames.” (Section 6, On Supervenience)

Vader Poster/Lucasfilm
This is the exact poster I had in my dorm room in 1998.
Source: Vader Poster/Lucasfilm

In philosophy of mind, and on most naturalistic understandings of reality, it is common to hold that the relationship between the brain and mind is supervenient. Mental activity is a result of brain activity, and no change or activity on the mental level is possible without a change or activity in the neuronal level. The mind supervenes on the brain.

This is controversial to some because Plato and Descartes’ view of mental activity, as something housed in a non-material entity called the soul which can separate and float away from one’s body when one dies, has been dominant in the western world for centuries. Substance dualism, as it is known—the idea that persons are made of two separable substances, one physical and the other mental—is the basis for many people’s understanding of human nature. It lies behind most people’s understanding of the afterlife, motivates people’s belief in ghosts, and even fuels their misunderstanding of personality and mental disorders. Addiction, for example, can’t be an illness if substance dualism is true. “If a person wants to quit smoking or drinking, they can just decide to do it. A decision can be made in the soul and reach down from outside the physical world to cause the desired behavior. You just have to have enough mental fortitude; it’s ‘mind over matter’.”

Our discovery that this is false—that the mind is not a separate entity but instead is something that depends on the brain—was the result of a series of discoveries which have revealed that specific mental operations are dependent upon brain activity; we saw that the former couldn’t exist without the latter. In some cases, we’ve mapped out what parts of the brain are active when certain sensations are present. (In this way, we have discovered that sensations of touch supervene on what is known as “The Penfield Map.” ) In other cases (e.g., Phineas Gage’s), we have observed what mental operations are missing when certain parts of the brain are damaged.  Memory, emotion, sight, smell, hearing, language formation and understanding—although we don’t have a full account of everything that is going on, we know which parts of the brain (hippocampus, limbic system, Broca and Wernicke’s areas, etc.) are responsible for such experiences and mental operations, and that they cannot exist without them. Everything that we used to think “went on in the soul” is now known to be dependent upon brain activity.

This has changed our understanding of ourselves and led to many scientific advances: treatments for addiction, corrective brain surgeries…the list goes on. And it also led many to accept that there is a supervenient relationship between the mind and brain. The obvious dependence of the mind on the brain seemed to clearly indicate a that you couldn’t have a change in the mind without a change in the brain. To be clear: things happen in our brains all the time of which we have no mental awareness; they do not produce any change in our mental states. But (so the theory goes) no change in our mental states can happen without a corresponding change in our brain states.

Why I moved this argument to Psychology Today

Whether the mind/brain supervenience hypothesis is actually true is irrelevant to my debate with Reppert. That’s why I moved my arguments on this matter to my blog.

Why is this issue irrelevant?

Reppert (via the argument from reason) claims that naturalism cannot maintain that mental processes (like the recognition of ground/consequent relations) are causally operative “on the basic level.” They cannot be the ultimate explanation for, say, why the naturalist concludes that naturalism is true. For the naturalist, Reppert says, all the real causal work is done by blind non-mental physical forces—like the firing of neurons. Such processes cannot generate truth reliably, Reppert argues…and thus on naturalism, the naturalist is not justified in believing that naturalism is true.

The astute reader likely has a million objections running through their mind at this point and, indeed, the argument fails for many reasons. But one big reason is that there are, in fact, naturalist theories that maintain that mental processes are causally operative on the basic level; and naturalist theories that maintain that there is a supervenient relationship between the mind and brain are among them.


Well...C. S. Lewis argued that, on naturalism, the mental was not causally operative at the basic level because, on naturalism, the mental could be subtracted from the world without changing it. But this is not true on varieties of naturalism that subscribe to mind/brain supervenience, since, according to mind/brain supervenience, there can’t be a change in the mental without a change in the brain. To subtract the mental from the world you would have to change everyone’s brain—which, obviously, would make the world a very different place.

Now, mind/brain supervenience might be false—but that doesn’t matter. The issue is whether or not naturalism necessarily entails that the mental is not causally operative on the basic level. Are there any naturalistic theories of mind that hold that the mental is causally operative on the basic level? Since some naturalistic theories of mind hold to mind/brain supervenience, and thus that you can't subtract the mental without changing the physical, there are some naturalistic theories which hold the mind to be causally operative on the basic level. And not even proving mind/brain supervenience false would change what those theories say. Thus the primary assumption of Reppert’s argument—that naturalism is incompatible with the mental being causally operative on the basic level—is false, and his argument fails.

Reppert’s Objection

But still, the issue of whether mind/brain supervenience is true is interesting…as is the very related issue of whether we could prove it scientifically. But in his reply to our original debate, Reppert suggests not only that we have not demonstrated it, but that we cannot.

"It is puzzling to me how a modal claim like supervenience can be supported by science. Some modal claims can be defended by science, when we discover identities between, say, Hesperus and Phosphorus [i.e., the morning star and the evening star]. Here two of the same kind of thing are shown to be identical and if identity is necessary, then science does discover some necessary truths. But mental states and physical states are not observable in similar ways, and the claim here is not an identity claim. So how could science confirm it? For neuroscience to have proven supervenience as a scientific fact, our mental states and physical states would have to be observable in the same way. For example, we perceive Hesperus and Phosphorus in the same way, and connect the two scientifically one identical star [they are both the planet Venus]. But mental states are not observable in this way." (Section IX). 

Here Reppert is invoking, without mentioning, the problem of other minds which suggests that we could never know that any mind beyond our own exists because only our own mind is observable to us. If we can’t even know (or scientifically prove) that the minds of others exist, the argument goes, how could we know (or scientifically prove) that minds supervene on brains? Wouldn’t we have to observe one always occurring in conjunction with the other, such that mental changes are always accompanied by brain changes? If we can only observe our own, how could it be possible to accomplish this?

(Philosophy of) Science to the Rescue

But scientific reasoning can solve the problem of other minds and, in turn, might also be able to demonstrate a dependence and even a supervenient relationship between the mind and brain. Reppert doesn’t realize this because his understanding of scientific reasoning apparently echoes Robert Almeder’s, who he quotes in defense of this idea:

“After all, where in the scientific literature, biological, neurobiological, or otherwise, is it established either by observation or by the methods of testing and experiment, that consciousness is a biological property secreted by the brain in the same way a gland secretes a hormone?” (Section IX, emphasis added.)[i]

The mistake here is thinking that science consists merely of observation and testing—that the only way science establishes anything is by performing an experiment. This is false. Indeed, as Ted Schick and Lewis Vaugn point out in my favorite critical thinking textbook (How To Think About Weird Things, 2014) the “observe, hypothesize, deduce, test” textbook definition of scientific reasoning is woefully inaccurate. (p.  161) They demonstrate what Ernan Mcullin also points out in “The Inference That Makes Science”…that all scientific reasoning is, at its base, abduction—inference to the best explanation. When one does science, one compares multiple hypotheses, and then accepts the one that is most adequate—in other words, the one that is (all things considered):

(a)    more fruitful (i.e., the one that makes the most correct novel predictions)

(b)   simplest (i.e., the one that requires the fewest assumptions)

(c)    widest scoping (i.e., the one that explains the most)

(d)   most conservative (i.e., the one that coheres best with what is already well established).

Now, to be sure, experiment and observation are often done to compare hypotheses—that’s how you determine fruitfulness—but it need not be. If there is not a test that one can perform to delineate between two hypotheses (thus showing one to be more fruitful than the other), we can still decide between them scientifically by comparing them according to simplicity, scope, and conservatism.

Take for example when heliocentrism was first proposed. Initially, there was no way to delineate it observationally from geocentrism. Both theories made the same predictions about where we would see the planets and we did not have telescopes powerful enough to observe parallax (the slight shifting of stars at different times of the year). And yet we still accepted heliocentrism—mainly because it was simpler and more wide scoping. (Finally observing parallax, with more powerful telescopes, just confirmed what we already knew.) 

Through exactly this line of reasoning, we can know that the minds of others exist. Although I cannot observe their minds, I can observe the existence of my own. I can see it directly correlated (and even seeming to cause) my own behavior, observe others behaving in essentially the same that I do, and then rationally infer the existence of (and be justified in believing in) their minds. Why? Because others having minds is the simplest, most wide scoping, conservative explanation for their behavior.[ii] Can I deductively prove it? No. But science is not about deduction; it’s about induction.[iii] Granted, this means that it guarantees nothing. But since knowledge only requires justification (not certainty), it doesn’t need to—and I can therefore know that other minds exist.

And once I do, the same kind of reasoning could likely establish a dependent relationship between the mind and brain. The best explanation for why similar brain damage in humans leads to similar mental deficits, and why the same areas of human brains are active when they report similar mental states, is because the brain is responsible for mental activity; the mental can’t exist without it. Once brain function stops, so does mental function. Along the same line, it seems that a similar argument could be given for mind/brain supervenience. That there can’t be any mental difference without a neuronal difference would seem to be the best explanation for what we have learned about the brain so far. We might even be able to determine what kinds of brain activity different kinds of mental activity supervenes on.

Now, to my knowledge, no one has done this…and do so would take some work; one would have to generate some competing hypotheses, compare them according to the criteria of adequacy, etc. And maybe such a venture would fail. But if it did, it wouldn’t be (as Reppert suggests) because the mind and brain are not “observable in the same way.” Again, that wrongly limits scientific reasoning to only observation and testing. If it did fail, it would most likely be because of a tie among the competing theories—"all other things being equal, one is simpler but the other has wider scope, and there is no way to tell which criteria is more important and thus decisive."

But what about the Churchlands?

If such thing could be scenically proven, Reppert wonders how can “neuroscientifically minded philosophers like the Churchlands propose eliminative materialism [the idea that minds don’t exist] without denying what is directly and scientifically observable”? Again, science doesn’t just traffic in “observation,” and such a scientific demonstration has yet to be accomplished—but this comment demonstrates yet another misunderstanding of how science works.

As Duhem, Quine, Popper and Kuhn all taught us, observation is theory laden. The observations we make, the tests we run, and the theories we form, are always informed by background assumptions. When we preform an experiment, or compare hypotheses, there are number of assumptions already at play. Consequently, if an experiment does not go as a hypothesis predicted, we have two options: we can reject the hypothesis, or we can change the background assumptions to bring the hypothesis in line with the experimental results.

Sometimes this is rational. For example, when Newton’s laws couldn’t accurately predict the orbit of Uranus, instead of rejecting them, scientist changed their background assumption about the number of planets. They hypothesized an eighth planet that was pulling Uranus off course—and that’s how we found Neptune.

Sometimes it is not. To borrow an example from Schick and Vaughn; you can see evidence that the Earth is round by watching ships sail off into the distance; the bow disappears before the mast as it dips over the horizon. But if you change your assumption about how light travels—instead of in straight lines, it dips down toward the earth—you can protect the flat earth hypothesis from falsification.

Regardless, this is all the Churchland’s would be doing if they denied mind/brain supervenience even in light of scientific evidence to the contrary.[iv] One of the background assumptions in such an argument would be that our observation of our own minds, and our awareness of their existence, is accurate. If you reject this assumption, and maintain instead that such an awareness is an illusion (as essentially the Churchland’s do), you can protect the “minds don’t exist” hypothesis.

Is this rational? On the one hand, our brain does deceive us in many ways; many of our perceptions are inaccurate. Could it be that our perception of our own mind’s existence is inaccurate? On the other hand, such excuses are definitely irrational if they are unfalsifiable. (Unfalsifiable excuses to save one from the evidence are called “ad hoc” excuses.) And it seems that one could make the case that the Churchland’s excuse is ad hoc. 

I’m not sure if such an argument would be successful, but it doesn’t matter for the point I’m making. The fact that the Churchlands could do this doesn’t mean that we can’t establish a supervenient relationship between the mind and brain beyond a reasonable doubt—especially since the background assumptions that they would likely ask us to deny are accepted by most.

Think of it this way. The fact that flat earthers can deny background assumptions to defend the idea that the Earth is flat doesn’t mean that science can’t establish that the Earth is round. In the same way, the fact that the Churchland’s can deny other assumptions to protect the idea that minds don’t exist doesn’t mean that science can’t demonstrate the supervenient relationship between the mind and brain.[v] This fact that they could just reminds us about the inductive nature of science—its conclusions are never certain. The fact that people can take advantage of this uncertainty to save their theories from complete falsification doesn’t entail that science doesn’t demonstrate what it does.

Testing Supervenience with Duplicate People

Reppert also claims that the supervenient relationship between the mind and brain could never be demonstrated because “…as Crane and Mellor pointed out long ago…we have no examples of two physically duplicate people to work with. A person physically identical to me will have the same mental states, according to supervenience. What kind of experimental evidence could we possibly supply for that claim?” (Section IX) 

Yet again, a fundamental misunderstanding of scientific reasoning is at work. It’s quite true that the supervenience hypothesis predicts that physically identical people will have indistinguishable minds. But this is only one of many predictions that the hypothesis makes and thus only one of many ways of testing it. It also predicts that human mental operations cannot happen without brain activity, that similar brain damage in different patients will affect their minds in similar ways, that medications that increase certain neurotransmitters will generate specific mental improvements, etc. All of these predictions have already been borne out.

Yes, two physically identical specimens having qualitatively identical mental lives would be a great piece of evidence for mind/brain supervenience—the final nail in substance dualism’s coffin so to speak. But I do not need such grandiose evidence to know that there is a supervenient relationship between the mind and brain any more than I need a trip to the International Space Station to know that the Earth is round. Yes, it would make it nearly certain—only the craziest of crazy ad hoc excuses could save the old way of thinking at that point. But if it could be demonstrated beyond a reasonable doubt, such wild grandiose proof would not be necessary.


As I stated in the debate, the concept of weak mind/brain supervenience is all the argument I made in my debate with Reppert requires. It would mean that mental operations, like reasoning, are causally operative in the actual world. They are not a “triffle” (as Reppert and Lewis claim), because you cannot subtract them from our world without making physical alterations to the brain. Since many naturalistic theories of mind hold to weak mind/brain supervenience, naturalism allows for mental operations to be causally operative on the basic level, and the argument from reason fails.

But there is also a view called “strong mind/brain supervenience.” It maintains that such a relationship holds between minds and brains in every possible world. In the same way that there is no possible world in which the frames of the Star Wars Trilogy are arranged as they are in the picture above but don’t look like Darth Vader, there is no possible world in which the matter of brains are arranged as they are but don’t give rise to minds. If this is true, then so called “philosophical zombies”—creatures with brains just like ours that have no minds—are impossible. They exist in no possible world.[vi]  

Can strong supervenience be proven scientifically? That remains to be seen. On the one hand, how could you prove something scientifically about all possible worlds? On the other hand, as I have shown, the fact that something is not observable does not limit our ability to draw conclusions about it scientifically. Could strong supervenience be the simplest and most wide scoping hypothesis once weak supervenience is established? After all, “everyone in this world that has a brain like me also has a mind like me” is the solution to the problem of other minds because it is simplest, widest scoping explanation. In the same way, could ‘everyone in every world that has a brain like me had a mind like me” be simpler and more wide scoping than the hypothesis that some worlds contain philosophical zombies?

Regardless, some kind of mind/brain dependence has clearly been demonstrated, scientifically, by neuroscience, beyond a reasonable doubt. This is enough to render belief is substance dualism (and souls) irrational.  Establishing supervenience scientifically would simply establish this dependence even further. That is why such a notion has been so vehemently opposed by substance dualists.  

A special thanks goes out to Ray Elugardo and Brint Montgomery who gave me useful feedback on these arguments.  

[i] It’s worth noting that no naturalist I know insists that the mind is (or is anything like) a hormone. An analogy I have heard used is that the brain produces the mind like certain mental produce a magnetic field.

[ii] The uniform assumption that all brains produced minds is simpler than the arbitrary assumption that some do and some don’t. The idea that mine does and others don’t raises unanswerable questions about why some do and others don’t, reducing its scope. And the uniform assumption also aligns with the uniformity we see elsewhere in nature.

[iii] Deductive arguments are supposed to guarantee their conclusions. Inductive arguments are supposed to provide probable support.

[iv] Interestingly, on eliminative materialism, mind/brain supervenience is vacuously true. There can be no change in the mind without a change in the brain if minds don’t exist.

[v] To be clear, I am not an eliminative materialist; but I am also not claiming that the Churchlands are the philosophical equivalent of flat earthers. Their arguments are much more sophisticated, and they might claim that it’s those who believe mind exist that are like the flat earthers; they are fooled by the way things seem.

[vi] On weak supervenience, philosophical zombies can exist in another possible world—just not in ours. For more on the difference between strong and weak supervenience, see The Stanford Encyclopedia of Philosophy