Substance Dualism v. Brain Functionalism

Haven’t posted in a while, I know. But I worked really hard on this and I hope you all enjoy it! It is sectioned off and labeled for easy reading, hopefully you don’t mind that! Citations refer to the works cited at the end! 


The mind, with which most people identify their individuality, is a seemingly innocent idea. It makes us who we are, and identifies others in the same way. However, under closer inspection, the mind becomes a very complicated entity. Is it a physical entity or immaterial? Is it an entity at all or can it merely be reduced to the brain? These are all questions that theories of the mind attempt to answer in their study of the mind. From René Descartes to contemporary philosophers, there have been many theories; some have lingered and others have nearly disappeared due to lack of support. Two that will be discussed here are Substance Dualism (SD) and Brain Functionalism (BF). The former being dualism, where as the latter is most often a type of materialism. Both theories have their strengths and weaknesses, as well as explaining the same issues in very different ways, which will be discussed in sections to come.


Substance Dualism (Including Strength & Weakness)

            Substance Dualism acquired its name from the separation of substances defined by René Descartes. Descartes defined the mind as a “mental substance” and the body, and other physical objects, as a “material” one. He then went on to further explain the differences of the two substances; material objects are spatial, “they occupy a region of space, excluding other bodies from that region” (H 18). They also have a specific shape, and maintain spatial dimensions. Contrary to material objects, mental objections (which for Descartes includes thoughts, senses, etc.) are non-spatial. It seems that this is a fairly reasonable presumption, as when one thinks of how thirsty they are, that thought does not come with necessary spatial dimensions, but rather is invisible to them, but they know it is there.

            A second difference between these two substances is qualitative, “Qualities of conscious experiences appear to be nothing like qualities of material objects. More significantly, they are apparently altogether unlike the qualities of any conceivable material object” (H 19). From this it seems the obvious conclusion is that mental and material objects are not of the same kind, their qualities are completely different. A third distinction between the two substances is the issue of privacy. Material substances are readily conceivable to anyone, however a mental substance is seemingly private to the owner of the mind. This also seems very reasonable; as two people can see the same movie, but one’s mental activity during that viewing is private due to the nature of the mind.

            In addition to distinguishing between mental and material substances, Descartes explained the relationship between properties and substances. He believed material substances (not in the modern sense of substances, but rather a substance being a tree, table, cup, etc.) had properties, and these two necessitated the others. “You could not peel off an object’s properties and leave a bare unpropertied substance. Nor could properties float free of substances and persist on their own” (H 22). These properties, for Descartes, are “modes of extension”. While material substances can have these modes of extension, Descartes makes the clear averment that mental substances cannot “extend”, thus cannot possess these modes of extension.

            While SD makes a sharp distinction between mental and material substances, it does recognize the clear interaction between the mind and body. The body is lead by the mind’s thoughts and desires, while the mind is also impacted by physical events that the body endures:

Imagine that you sit on a tack planted in your chair by a malicious practical joker. Your sitting on the tack (a material event involving a pair of material substances, the tack, and your body) gives rise to a distinctive sensation of pain (a mental event). This sensation or feeling in turn generates another mental event, a desire to leap upwards, and this desire brings about a leaping. (Heil 23)

This view of mind-body causal interaction is quite compatible with how common sense perceives it, but this is where SD encounter’s its main weakness. If the two substances, mental and physical, are of completely different kinds, how does Descartes suppose they interact? Descartes explained the causal interaction between the two as making contact in the pineal gland in the brain. This does nothing to solve the problem, however, because it poses the same problem of the mental and physical interacting without any sort of transfiguration of either. This is a major weakness of SD because without a formal explanation for how mental and physical substances interact, the entire theory loses much credibility.

        While this problem of causality may seem like it cannot be overlooked, SD does have its strengths. The most prominent strength seems to be that the mind and body do, in fact, seem to be of completely different kinds. While material substances are those that we can sense, and are readily available for others to sense also, mental substances are of a completely different variety. Not only are they of different kinds, but they are also different in privacy. One cannot know another’s thoughts in the same way the owner of those thoughts does, but rather only in a second or third-hand way. Thus, SD seems to be one of the only theories of the mind that can explain why the mind is so apparently different from the body and other material objects.


Brain Functionalism (including Strengths & Weaknesses)

            Brain Functionalism (BF) is the theory that holds the mind is the way the brain works, or functions. Functionalism emerged in the mid-1900s, mainly rising in response to the Identity Theory and Behaviorism. Functionalism is the theory of mind that most people hold today, and is generally a materialistic view, though that is not necessarily true. However, in the interest of this paper, BF will be considered a materialist theory. BF holds that mental events (thoughts, sensations, desires, etc.) are not reducible to specific entities or even brain events, but rather they are how we understand how the brain works:

Functionalism in the philosophy of mind is the doctrine that what makes something a mental state of a particular type does not depend on its internal constitution, but rather on the way it functions, or the role it plays, in the system of which it is a part. (Functionalism, SEP)


Rather than identifying a specific entity that is the mind as SD does, functionalism explains the mind as the way we understand the brain; the mind is a collection of mental events.

            This theory of the mind is much more beneficial to contemporary sciences, as the mind becomes much more inclined to study when we understand it as what it does rather than what it is. It is agreed upon by functionalists that we cannot directly observe mental events, not even the owner of those events.  Because the way we know our thoughts is through words, not by what they are composed of. (BR) While functionalism as a whole is neutral on the matter of materialism and dualism, Brain Functionalist claims that “the brain that produces mental events. It is the brain that thinks and remembers and deliberates, for example. This does not mean that the mind is the brain. The brain functionalist denies that. Instead, the mind is simply the way that the brain functions. It is what the brain does. It is the collection, if you will, of all the software or programs run by all the modules of the brain.” (BR).

            An important distinction, already hinted at, is that BF insists that we do not know the intrinsic nature of mental events, but rather by their causes and effects. The role of mental events is to link environmental inputs with the subsequent behavior. This theory makes great strides in the problem of minds in other forms (animals, machines, silicon, etc) because by not having a strict identity of what the mind truly is, as the Identity Theory does, BF maintains that minds are multiply realizable. This means that because the mind is merely the way the brain functions, that the mind could be “realized”, or function, in similar ways in animals, machines, other elements, etc. This is because the mind can be seen as a software program of the brain, and just as we know Microsoft Office can be opened on an Apple computer as well as a PC, so can a mind hypothetically be “realized” in other forms than the human brain.

            Another fundamental claim of BF is that the mind is modular. By this BF means that because the brain functions in a modular fashion, then the mind does, as the mind is the way the brain functions. This means that the mind is not one entity, but is composed of different modules:

Each mental state is produced by a host of separate functions, each doing its own thing independently of the other functions. The brain is wired less like a digital computer, where steps are carried out in one long sequence, and more like parallel processors, which run smaller programs simultaneously and then weave together their results to produce a finished product. It appears that “the” mind is really lots of separate “minds”, each of which does its own job in isolation from the rest. Then, somehow, the results get pooled with all the others to form a mental event (BR)


This theory of the mind poses a few problems for the common sense way of imagining the mind. First, this means that the mind is a collection of complex functions, rather than being one entity, which brings us to the second problem. If the mind is modular, then this means that perhaps we do not have the “self” we are all so quick to assume that we do.  The third problem of the modularity of the mind is that of knowledge and reality. “It is no longer possible to think of our knowledge of the world as a ‘copy’ of the external environment in any sense.  Rather, it appears that Kant was on the right track. As processors of information, our minds now have to be thought of as ‘grinding up’ data received from the environment and transforming it into the sorts of information that ‘fit’ the processors of the brain.” (BR).  These are not the weakest parts of the BF view, however, as they do not pose much of a scientific problem so much as they bother the common sense conception of the mind most people hold.

            The ay BF describes the processing of input from the environment can be seen in an analogy to a computer. Just as one can describe the inner workings of a computer in different ways, so can one describe BF. There are three stances to take when describing the way something works: intentional, design, and physical. The intentional stance is the goal/purpose of the entity. The design stance is the way an entity works, and the physical stance is the material that entity is made of. Thus, we can talk about the mind in its physical sense, which is how neuroscientists would approach the problem. We could also look at it in how it works on the design level in how the mind functions, or the intentional level and study the goals and purposes of the mind.

            The biggest problem of BF is that of “qualia” or consciousness. This problem will be discussed in more depth in a subsequent section, so it will only be touched upon here. BF does not account for how people truly experience things, such as hearing beautiful music or smelling springtime in the air, as surely these are senses with emotion attached to them. This would suggest that a zombie is comparable to a human being, and surely we can all agree that we have qualitative and conscious experience enough to not consider ourselves zombies.

            The most prominent strength of BF is that the theory allows for minds that are multiply realizable, or that they can be realized in a wide array of physical states, rather than merely the human brain. This is a problem BF solved, for Identity Theory especially; in that no matter the physical nature of an entity (whether it is an alien made of silicon, a computer, etc.) a creature can have a mind and experience the same functions a human does. For example, a coffee mug is typically thought of as a cylindrical piece of ceramic that holds coffee. If this were the only “realizable” coffee mug, then what of a metal travel mug? Even hands could “realize” the function of a coffee mug. Brain functionalism gets rid of the type-type identity problem in that a specific entity is no longer the only thing that can carry out a function. With the BF, the ceramic mug and travel mug can both hold coffee because they perform the same function.



            The problem of intentionality is the problem that mental events, thoughts, have an “about-ness” to them. When someone thinks of a chocolate bar, the thought is “about” chocolate. It seems that only mental events have this intentionality, for no physical object can have an about-ness to them. In this topic we find a fairly strong argument for Substance Dualism. By separating mental and material substances, the Substance Dualist can more readily explain why there is such a difference between mental events and physical ones. Because of the fact that, for a Substance Dualist, the mind and body are of completely different material, or lack-there-of, this explains why there is no physical parallel counterpart to the fact that there seems to be an “about-ness” to thoughts.

            Brain Functionalism does not do as well when it encounters the problem of intentionality. Brain Functionalism suggests “that intentionality requires an incoming casual component of some kind”, however this does not account for any thought that does not have a “causal component”. Heil explains further:

Although perceptual experience undoubtedly recedes (and has a causal bearing on) subsequent reflective imagistic thought, it is not this causal linkage that accounts for thoughts’ projective character. Projectivity is built into the thought. (Heil 253)


BF seems to ignore the fact that thoughts themselves have this “about-ness”, and instead insist that all thoughts that seem to have intentionality in fact just have a causal component that directly caused that. This would mean that thinking of how much you loved a movie seen years ago would necessitate a cause for thinking about that specific movie, since intentionality pertains to the about-ness of thoughts.



            Substance Dualism again harkens back to their claim that mental and non-mental entities are of complete different kinds to explain consciousness. SD says that consciousness is possible because states of mind are “ ‘directly available’ only by the person (or creature) to which they belong” (H 20). Descartes argument for consciousness is that because mental states are only present when we introspect, and all mental states are states of the soul. Therefore, we can conclude from this that we are able to be conscious of the soul, thus ourselves.

            This is a fairly weak argument for consciousness, however. What of the “mental states” that are not apparent to us? If we are not “aware” of enough mental states, would we not be conscious? Substance Dualism suggests that because we do not know, and cannot know, the true nature of the mind, like we do physical entities, then we will never know the true nature of consciousness. The support for consciousness that SD provides has many holes, and relies completely on the separation of immaterial and material entities, which means to accept this theory of consciousness, one must accept the entire theory. Needless to say, this theory of the mind is rarely supported as of late.

            Today most theories of the mind held are materialistic. However, even these theories do not have everything figured out. BF maintains that somehow physical and mental states combine to form consciousness. For the Brain Functionalist, a conscious brain state would go as follows:

You would be in a state brought about by tissue damage, or pressure, or extremes of temperature, and one that itself brings about a range of characteristic ‘pain responses’, behavioral and mental: you cringe, you form the belief that you are in pain, and you acquire a desire to take steps to alleviate the pain. (Heil 164)


We conclude from this statement that consciousness is the causal interaction between mental and brain states. This does not quite explain consciousness, however. This is the most significant weakness of Brain Functionalism. BF seems to explain many other problems of the mind very well, but consciousness is the most prominent problem that theories of mind attempt to solve, as this is what all people identify themselves with; it is their individuality. Surely there is more than the mere causal interaction between mental and physical events. And this is why BF does not do well in explaining intentionality, as the two are deeply intertwined.

            If we are to adhere to a f BF view, then by this line of thought, a zombie would be the functionally (and altogether) identical to humans. By definition for BF, the mind is just how the brain works. If there were a zombie, which is an unconscious human, then due to the inability to explain consciousness, BF is essentially stating that we would be identical to a zombie, when we clearly are not.

            Another problem we run into when we BF attempts to explain consciousness is the problem of “emergence”. While it is easy to see that “by putting together molecules in the right way, you get a liquid. But how could you get consciousness from arrangements of nonconscious material particles?” (Heil 172). How can the feeling of very much enjoying a banana split, and subsequently desiring one, be reduced to a physical event in the brain? Brain Functionalism falls short with their explanation of consciousness here.


Free Will

            Free Will is an aspect of human life that most people want to know they have. The Substance Dualist believes that humans have free will, but again by relying on the fact that there are immaterial and material substances. SD asserts that because of this duality, the mental substance does not have to abide by natural laws, laws of physics, whereas physical substances do. Because mental substances do not abide by natural laws, the mental substance (mind) is free to do as it pleases, thereby giving people free will. This satisfies the Substance Dualist view, but does not satiate the Brain Functionalist.

            BF does not give a clear-cut answer to the problem of free will. However, with materialism always comes some sort of determinism. Determinism holds that people do not have any free will, but are at the whim It seems that the mercy of the laws of physics. Brain Functionalists would most likely take a compatibilist view. Compatibilism, also known as soft-determinism, argues that while events of the universe are out of our control, there are still opportunities to make decisions:

For my ordering tea to be a free action, it must have been possible that I could have ordered something other than tea. And even though the laws of physics are deterministic or probabilistic, it was possible for me to order something else: There were other options on the menu; I was not at that crazy café where a customer’s order is determined by a toss of a coin; I had enough money with me to order something more expensive than tea; I’m not addicted to tea and so could have chosen something else; and so forth. (Montero 133)


Even if the nature of the universe is not in our control and subject to laws of nature, the Brain Functionalist would argue that we do have a certain amount of free will. Because mental events are merely the way the brain works, and not identified in the type-type strict identity fashion, the laws of nature may not effect the way our thoughts, consciousness, thus decisions are formed.


Personal Identity

            Personal identity, at first glance, does not seem to be a problem. The fact that we are the same person as we were when we were younger seems to be unquestionable, but how do we know? How do we get gain the sense of personal identity? SD’s first, and weaker, argument in the case of personal identity for SD is that of memory. This argument maintains that we can link our personal identity through memory. Even though we may not remember everything that has happened in our life, there is a memory we have of ourselves which is linked to memories we may no longer have.

            The second argument Substance Dualism contends is that a person maintains their personhood through change due to the mind they possess throughout. This, again, requires the acceptance of an immaterial mind that comes with Substance Dualism. This sufficiently argues why, although our body is losing and replacing cells throughout our entire life, our sense of personal identity remains true. SD says we can only know ourselves through introspection, and even then there are still some mental states which we cannot know. Some may say that this is reason to doubt Substance Dualism’s idea of the immaterial personal soul all together, but there are other instances of not knowing entities which we do accept:

We can still reasonably believe that minds exist as theoretical entities. This requires a bit of explanation. There are many sorts of entities that science claims to know,  even though they cannot be experienced directly. Think about the subatomic particles studied by physics, for example. They cannot be observed, because the most powerful instrument for making observations of small objects is an electron microscope. (BR)



The Substance Dualist would argue that because there are other instances in which we do not directly experience something, but still accept its existence, this is not a valid counterargument against Substance Dualism’s sense of self, and soul in its entirety.

            The Brain Functionalist takes a similar view to the Substance Dualist in saying there is a psychological-continuity. There is a unity we have as individuals in that the mental states and the causal roles thereof all must come from the same being:

What the functionalist view claims is that it is of the essence of a mental state to be cause in certain ways, and to produce, in conjunction with other mental states, certain effects (behavior or other mental states). But of course, it is in conjunction with other mental states of the same person that a mental state produces the effects it does; and its immediate effects, those the having of which is definitive of its being the mental state it is, will be states (or behavior) on the part of the very same person who had the mental state in question. (Olson 683)


Each mental state is coming from the same being, and because of this we form a sense of unity amongst our mental events. In order for the Brain Functionalist to make a case for personal identity, all that is needed is the same being producing mental events that combine over time, thus creating a stream of psychological continuity.

            A counterargument for both of this BF argument is the “brain-switch” thought experiment. There is no actual answer to what would happen if there were to be a brain transplant, but suppose your brain were switched with another’s. Would you stay in your own body, or would you now inhabit the body where your brain has gone? Most say that the body in which their brain now resides is where they would be. However, if the mentioned BF argument for personal identity is valid, then the same animal is necessary to have the same being contribute to mental states, as the mind is just how the brain works, and this would mean the body is an imperative part of the self.


Other Minds

            The problem of other minds is simply, how do we know others think anything like we do? How do we know other persons’ minds even slightly resemble our own? The Substance Dualist answers this question with an analogy. The analogy is as follows, “I can extrapolate from my own case. I know that certain of my mental states are correlated with certain pieces of behavior, and so I infer that similar behavior in others is also accompanied by similar mental states.” (Dualism, SEP).  This is a fairly weak argument for the proof of other minds, as it is an inductive argument working off of just one observation.

            Brain Functionalism takes a very similar route in explaining other minds, however it is a bit more convincing than concluding an inductive argument from one observation. BF uses an analogy as well, but instead of using behavior, it uses the existence of the brain:

Mental states are conceived of as inner states which are the means by which an organism responds to its surroundings. The different mental states are characterized by their various roles, their typical causes and effects. They are in this way alone distinguished one from another. So a burning pain is that inner state typically caused by being burned and typically leading to wincing and crying out and such like behavior. It follows that all that is required to reach the conclusion that other human beings have such inner states is merely careful observation of how they behave and in what circumstances. (Other Minds, SEP)


By having billions examples of people possessing brains and exhibiting similar behavior (thus, functions), we can conclude that others have similarly functioning minds, as it is merely how the brain functions.


Life After Death

            In addressing the issue of life after death in terms of Substance Dualism, we are again reliant on the sharp distinction between mental and material substances. Because the two are of completely different kinds, one does not necessarily depend on the other. Because the mental substance (mind) does not depend on the body to survive, SD allows for the possibility of the afterlife. This would adhere with most Substance Dualists’ belief in God, as this was a reason for Descartes himself.

            The Brain Functionalist is almost the exact opposite from the Substance Dualist. Because Brain Functionalism is a materialistic-functional theory of the mind, which means the mind is merely how the brain functions, not a separable entity as in SD, life after death seems impossible for the Brain Functionalist. There would be no “soul” , as a Substance Dualist might suggest, to move onto a life after death of the body. Instead of separating from the body after death, the mind would die with the death of the body, as they are integral to one another. As Olen suggests in the section on personal identity, to be you includes staying in your body, which would contradict the idea of the afterlife.

            The only way the afterlife would be possible in the Brain Functionalist’s point of view is if the mind could be “downloaded” somehow and “realized” in another body after the death of the former. Because BF holds that minds are multiply realizable, this idea would hold in some sense, but again brings us back to the problem of personal identity. There would need to be an extreme amount of reconciliation between personal identity and the means by which a person enters the afterlife for the two ideas to be compatible.

            Substance Dualism and Brain Functionalism are very different theories of the mind. While SD asserts that the mind and body are of complete different kinds, BF holds the opposite, that the mind is truly just the way the brain works. Both have their strengths and weaknesses, but the much more widely accepted theory today is Brain Functionalism. While it may seem that Substance Dualism has its explanations better defined, fact that one must accept without doubt that the mind and body are of two completely different substances comes with much difficulty for most people. While Brain Functionalism has its problems, it is most compatible with contemporary sciences and allows for a much more accessible idea of the mind.

Works Cited

Hyslop, Alec, “Other Minds”, The Stanford Encyclopedia of Philosophy (Fall 2010 Edition), Edward N. Zalta (ed.), URL = <;.


Levin, Janet, “Functionalism”, The Stanford Encyclopedia of Philosophy (Summer 2010 Edition), Edward N. Zalta (ed.), URL = <;.


Olson, Eric T., “What Does Functionalism Tell Us about Personal Identity?” (Wiley) Noûs, Vol. 36, No. 4  (Dec., 2002), pp. 682-698 <;


Robinson, Howard, “Dualism”, The Stanford Encyclopedia of Philosophy (Winter 2012 Edition), Edward N. Zalta (ed.), URL = <;.



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