Thomistic Substance Dualism Essay

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In an addendum to his article “Why I Am Not a Property Dualist,” John Searle suggests that property dualism really entails substance dualism. For it describes mental properties as “arising from” and existing “over and above” the brain, and “these metaphors suggest that… consciousness is something separate from the brain” given that “uncontroversial properties of the brain, like weight, shape, colour, solidity, etc.” are not said to exist in that way. For consciousness to exist “over and above” the brain requires that it be “a separate thing, object, or non-property type of entity.”

Searle’s claim here seems reminiscent in some ways of Aquinas’s argument for the subsistence of the human soul at Summa TheologiaeI.75.2:

Therefore the intellectual principle which we call the mind or the intellect has an operation "per se" apart from the body. Now only that which subsists can have an operation "per se." For nothing can operate but what is actual: for which reason we do not say that heat imparts heat, but that what is hot gives heat. We must conclude, therefore, that the human soul, which is called the intellect or the mind, is something incorporeal and subsistent.

Aquinas has argued earlier for the first claim made in this passage, viz. that intellectual operations do not involve a bodily organ. What he saying here is that this claim entails that that which carries out these operations, the human soul, must “subsist” apart from the body; it isn’t a mere accident or attribute of the body. The reason is the Scholastic principle that agere sequitur esse, activity follows upon being. Heat, as a mere accident or attribute, cannot cause something to be hot; rather it is the substance which has the heat that causes something else to be hot. Similarly, an operation which is not carried out by any bodily organ but which – qua operation rather than substance – cannot exist apart from some substance or other, must inhere in something immaterial.

So, when Searle tells us that immaterial mental properties would have to inhere in something immaterial rather than in a material object like the brain, he is saying something which seems to dovetail with Aquinas’s argument.

Still, things are a bit more complicated than that. For one thing, unlike many contemporary property dualists, Aquinas does not regard consciousness (as contemporary philosophers tend to understand “consciousness”) as immaterial. Rather, it is intellectual activity (grasping abstract concepts, reasoning, etc.) that he takes to be immaterial. Second, Aquinas is not only not a property dualist, he is not (contrary to appearances) a substance dualist either, certainly not in the way that sort of view has been understood since Descartes. Rather, he is a hylemorphic dualist. From a Thomistic point of view, substance dualism, property dualism, materialism, idealism, neutral monism, and all other post-Cartesian theories of the mind presuppose a mistaken and muddleheaded conception of both mind and matter – a conception which (among other things) makes it very difficult for contemporary philosophers even to understand the Thomistic view. As when dealing with Aquinas’s position on other specific philosophical questions, the only way properly to understand what he says about the relationship between mind and body is to situate it within his general metaphysics, which presupposes an understanding of the notions of act and potency, form and matter, substance and accident, essence and existence, analogical predication, etc. I set all this out in Aquinas, with chapter 4 devoted to Aquinas’s psychology and how it differs radically from contemporary substance dualism, property dualism, etc.

Now, some further reading, while you wait for your copy of Aquinas to arrive: First, my essay “Why Searle Is a Property Dualist,” which explains why Searle’s own anti-materialist arguments in philosophy of mind do in fact entail property dualism, despite his attempt to avoid this result. Second, check out David Oderberg’s article “Hylemorphic Dualism” for an overview of the Thomistic position. Third, take a look at Alfred Freddoso’s article “Good News, Your Soul Hasn’t Died Quite Yet” for a discussion of some of the differences between the Thomistic view and the standard modern ones (and why Catholics, especially, should be wary of the latter, including modern versions of dualism).

Case for Dualism

The following was a paper responding to the topic, “What are the major issues involved in the mind-body (or mind-brain) debate? Which position(s) do you consider most convincing, and why?”

Within this paper I create a case for a substance dualism without discriminating between the Cartesian and Thomistic forms. Discover why a physicalist solution to the mind-body problem fails. Read about some common criticisms against substance dualism, followed by my defense.

Scott D. Brisbane



In this paper I will look at the major issues within the mind-body debate, and then describe substance dualism highlighting important differences between two positions I find convincing—the Cartesian and Thomistic forms of substance dualism. The body of this paper will then:

  1. present reasons for favouring substance dualism;
  2. explore four main criticisms laid against substance dualism; and
  3. provide a defense to counter such criticisms.

This paper will finally conclude by summarising what has been covered.

Before proceeding, it seems important to be aware that distinctions are often made between the mind, soul, spirit, and the ego or self. Unless otherwise noted, they will be used interchangeably to more easily focus on the mind-body problem.

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Major Issues

Issues within the mind-body debate generally surround two predominant ones. The first concerns whether a human is made of only one component, such as matter, or two components, such as mind and matter. The second is, if there are two components and they affect each other, then how do they interact? Being aware of these issues, I now turn to briefly describe substance dualism and the varieties that this paper is concerned with.

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Substance Dualism

Substance dualism generally holds that the body is a physical object having physical properties and that the mind is a mental substance containing mental properties irreducible to the physical. When one experiences pain for example, the body may incur certain electrical and chemical stimuli (physical properties), which results in the self or soul consciously experiencing the felt quality and awareness of pain (mental property). Thus, it is argued within substance dualism that the soul and brain closely interact with each other, though they are different substances with differing properties.

In defending substance dualism, I will be specifically siding with the “essence” of the Cartesian and Thomistic forms, without further developments such as Parallelism and Occasionalism which attempt to provide solutions specifically for the mind-body interaction problem. Although it is beyond this paper to explore all the distinctions between the Cartesian and Thomistic sides, it is worth noting two.

Firstly, Cartesians tend to reduce the soul to the mind, whereas Thomists believe the mind to be a faculty of the soul that requires certain physical states to obtain in the brain and central nervous system before the mind can function. Being aware of these differences between the soul, mind and body, the second distinction is that Cartesians view the mind as a substance separate from the body which is considered to be a purely physical object. Thomists however, will insist upon a more intimate relationship where the soul provides the form or essence of the body. As such, Thomists believe that the soul can exist in a disembodied form, although as philosopher Scott B. Rae writes, “the body is an ensouled biological and physical structure that depends on the soul for its existence.” (emphasis mine)

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Arguments Favouring Substance Dualism

It should be noted that the arguments exposed below also favour “property” dualism, that is, one material substance having both physical and mental properties. These arguments are presented in order to demonstrate that the mind and mental states are distinct from the brain or physical states. If they are not identical, then dualism as the only other option must be true.

Due to word restrictions, this paper is unable to explore reasons why I am more convinced of substance dualism; however, let it remain I find substance dualism more convincing because I believe it better satisfies our sense of libertarian freedom (free will), and better explains our sense of a unified self enduring through time despite physical changes. In addition, substance dualism satisfies our sense of immortality, and serves as a cornerstone to providing real hope to our sense of ultimate meaning in life.

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Distinctiveness of the Mental And Physical

It seems obvious that physical properties do not have the same features as mental properties. For example, mental events such as thoughts, feelings of pain and sensory experiences do not contain physical qualities like mass, spatial dimensions and space location, are not composed of chemicals, and do not have electrical properties. As monist Keith Maslin summarises, “physical occurrences do not just appear to be different from consciousness; they are utterly different, so utterly different in fact, that it is inconceivable how the physical could produce the mental.”

In a published journal, Bruce Hinrichs points out:

When a person reads a sentence, hears a speech, experiences an emotion, or thinks a thought, a cluster or network of brain cells fires in a certain pattern with particular intensity and timing.

Likewise, it has been observed that when a part of one’s brain is touched with an electrode, it may cause a mental experience such as a memory to occur. Some might classify this as evidence that mental states are reducible to physical states, yet this only demonstrates that the mind is causally connected to the brain and not that they are identical. Therefore, the distinctiveness of mental and physical properties and states argues favourably of substance dualism.

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Self-Presenting Properties and The Subjective Nature of Experience

The existence of self-presenting properties and subjective states of experience also argues favourably of dualism. To illustrate, consider the following words by George Graham:

Perceiving a ripe tomato may mean one visually experiences red. There is a qualitative difference between how red looks to us and green. Coconut milk may offer a sweet taste and, once again, there is a qualitative difference between the taste of coconut milk and the taste of bitter beer.

Mental properties such as the taste of beer, having the thought that a tomato is ripe, or experiencing red, are what philosophers Craig and Moreland refer to as “self-presenting properties.” They write:

[Self-presenting properties] present themselves directly to the subject, they are psychological attributes, they are directly present to a subject because that subject simply has them immediately in his field of consciousness.”

They also highlight two pieces of evidence for claiming mental properties are self-presenting, while physical properties are not. The first consists of the “private access” someone has to their own mental life—a privileged first-person perspective of knowing their own thoughts and felt sensations. The second is being able to know some, if not all, of our mental states “incorrigibly,” that is, in a way we cannot be mistaken. For example, if one sees Graham’s ripe, red tomato, it seems impossible for them to be mistaken that they are consciously experiencing a red sensation. Yet, Science tells us what one perceives as a red tomato is actually a tomato which absorbs and reflects different wavelengths of light. This does not destroy the fact that a red quality was incorrigibly “subjectively experienced.” As Howard Robinson explains:

The notion of having something as an object of experience… does not figure in any physical science. Having something as an object of experience is the same as the subjective feel or the what it is like of experience. (emphasis mine)

In other words, states of experience have a subjective qualitative feel to them (phenomenal qualia), and this does not appear to be true of anything physical. As Maslin further illustrates:

All that is essential to pain is the way it feels. What may, or may not be taking place neurologically is irrelevant, since all that is required is that if a sensation seems to you to be painful, then it is painful… Because mental states are not necessarily identical with brain states, since we can genuinely imagine the one without the other, they are not identical at all.

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Secondary Qualities

Secondary qualities are said to consist of properties like colours, tastes, sounds, smells and textures, whereas primary qualities are properties that characterise matter such as weight, shape, size, solidity, motion. The fact that secondary qualities exist favours dualism, for as Frank Jackson explains, a strictly physical and material world arguably force us to deny them:

We sense the world as made up of coloured, materially continuous, macroscopic, stable objects; Science and, in particular, Physics, tells us that the material world is constituted of clouds of minute, colourless, highly-mobile particles… Science forces us to acknowledge that physical or material things are not coloured… This will enable us to conclude that sense-data are all mental, for they are coloured.

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Intentionality is often referred to as the “ofness” and “aboutness” of mental states, and is perhaps one of the strongest arguments favouring dualism. One physical object can be harder and larger than another to the left, yet it cannot be “of” or “about” another physical object because “aboutness” is not within the language of physics and chemistry. As Craig and Moreland illustrate:

[T]he fact that Joe’s thought that the painting is beautiful is about the painting cannot be reduced to any physical fact about Joe’s brain and central nervous system. Nor can this fact be identified with Joe’s disposition or tendency to do certain things; for example, to smile and say, ‘How lovely that is!’ after viewing the painting. If one’s thinking is nothing more than his being disposed to behave in certain ways, then one would have no idea what it was that one was thinking about until that behavior was manifested!

Many agree intentionality poses a problem for physicalist positions. Maslin writes, “intentional states are essentially holistic, but there appears to be no counterpart of this feature in purely physical states of affairs.” Dale Jaquette remarks, “because of its intentionality, the mind is qualitatively different from non-mental, purely mechanical things.” George Graham declares, “No physical phenomenon possesses anything similar. Intentionality is not physically constituted.”

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Criticisms of Substance Dualism

1. Problem of Interaction

The problem of the interaction between the mind and body is perhaps the leading criticism laid against substance dualism. As Maslin points out, “how can the soul, lacking all physical dimensions, possibly affect, and be affected by, the extended body?” There appears to be no reasonable way to explain how each separate substance could interact with the other.

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2. Argument from Evolution

Maslin comments, “Evolution gives us a reason for privileging the physical.” It is suggested that because life on Earth is the result of an entirely physical process, then humans are entirely physical beings. Paul Churchland forcefully drives this home:

The important point about the standard evolutionary story is that the human species and all of its features are the wholly physical outcome of a purely physical process… If this is the correct account of our origins, then there seems neither need, nor room, to fit any non-physical substances or properties into our theoretical account of ourselves. We are creatures of matter. And we should learn to live with that fact.”

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3. Argument from Ockham’s Razor

The principle of parsimony (Ockham’s razor) is often invoked against dualism. As John Heil explains, “Ockham’s Razor bids us not to ‘multiply entities beyond necessity.’” Given two accounts of the same thing, people should prefer the ones that are simpler, that is, “accounts that refrain from introducing new kinds of entit[ies] or process[es].” As such, a theory that only involves one entity (i.e. matter) to explain a human being, ought to be preferred over one involving two entities.

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4. Problem of Many Minds/Counting Souls

If the soul and body are different substances, then it is said this leads to a problem of ascertaining how many souls or minds are present within a body if they are not testable. As Maslin expresses, “How do you count souls? By what criterion or criteria is it decided at a given moment how many souls are present?” The point being made is why would we expect there to be only one mind attached to a body. As Craig and Moreland explain, “Since dualism cannot rule out the possibility of many minds, dualism leads to skepticism about our knowledge of how many minds others have, and thus it is to be rejected.”

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Defense of Substance Dualism

1. Problem of Interaction

The argument that we do not understand how a soul interacts with a physical body, appears to be based on an appeal to our ignorance (argumentum ad ignorantiam). For it assumes if we do not know “how” A causes B, especially if the two consist of different properties, that it is not reasonable to believe the two can interact. Yet, as Craig and Moreland point out, a tack can be moved by a magnetic field, and gravity acts on a planet millions of miles away. Gravitational forces and magnetic fields appear to have very different properties to the solid and spatially located entities they affect, and although we may not understand “how” such interaction takes place, it nonetheless does—just as we are alert to causation between the mind and body. As another example, even if one is not a theist, most do not view it as inconceivable to believe that God (given God’s existence) created the material universe and could act within despite each one being very different.

A second defense is that the question of “how” the mind interacts with the body may not even arise. As Craig and Moreland explain in depth:

One can ask how turning the key starts a car because there is an intermediate electrical system between the key and the car’s running engine that is the means by which turning the key causes the engine to start. The “how” question is a request to describe that intermediate mechanism. But the interaction between mind and body may, and most likely is, direct and immediate.

If the interaction is direct and immediate, as Thomists would tend to believe, then there is no reason to assume there is an intermediate mechanism that facilitates the interaction.

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2. Argument from Evolution

Given our current scientific knowledge, an argument from a natural, physicalist origin seems a bit premature and hollow. While many Scientists may accept biological evolution as a fact despite its mechanism(s) not being fully understood or settled upon, the “origin of life” continues to remain a mystery. As best-selling author and Natural Philosopher Paul Davies (an agnostic) writes:

When I set out to write this book, I was convinced that science was close to wrapping up the mystery of life’s origins… Having spent a year or two researching the field, I am now of the opinion that there remains a huge gulf in our understanding… This gulf in understanding is not merely ignorance about certain technical details, it is a major conceptual lacuna.

Yet, placing talks of biological origins aside, consider this evolutionary argument framed in the modus ponens form:

  1. If we are the result of naturalistic, evolutionary forces, then physicalism is true.
  2. Humans are merely the result of naturalistic, evolutionary processes.
  3. Therefore, physicalism is true.

Despite differing words, physicalism is assumed for both sides in the first premise, which clearly begs the question. Yet the dualist could turn the argument around to be more favourable:

  1. If we are the result of naturalistic, evolutionary forces, then physicalism is true.
  2. Physicalism is not true.
  3. Therefore, we are not the result of naturalistic evolutionary processes.

If evidence for a non-physical mind is good, then perhaps the latter modus tollens form of the argument should be embraced?

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3. Argument from Ockham’s Razor

As previously pointed out by John Heil, Ockham’s razor bids us not to multiply entities beyond necessity. While I would agree that one should not needlessly postulate dualism, the arguments presented for a dualist position include mental phenomena for which a purely physicalist theory appears inadequate to explain. As David Chalmers reveals:

These [physicalist] models and theories are simply not the sort of thing that could explain consciousness… Any account given in purely physical terms… will yield only… structure and dynamics. While this is enough to handle most natural phenomena, the problem of consciousness goes beyond any problem about the explanation of structure and function, so a new sort of explanation is needed.”

Therefore, the real debate is not about Ockham’s razor, but rather the explanatory power and relative merits of each position.

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4. Problem of Many Minds/Counting Souls

As substance dualism can allow for the possibility of many minds in one body, perhaps it should be seen as a virtue rather than a problem. For example, it would have greater explanatory power for certain phenomena such as demon possession or Dissociative Identity Disorder (aka Multiple Personality Disorder), something a non-dualist approach might have difficulties accounting for.

Secondly, Thomistic substance dualism, as earlier described, would justify the expectation of one soul to one body because its allows for a more intimate connection between the body and soul than its Cartesian rival.

Lastly, it does not following that all scepticism is reasonable, or that one can only know something when all scepticism is ruled out. Based on experiences with our own consciousness and the lives of others, it is reasonable to claim that one mind is attached to one body unless there is evidence to the contrary.

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Concluding Summary

The mind-body debate surrounds two main issues:

  1. Is a human being composed of matter only, or mind and matter?; and
  2. If both, then how do mind and matter interact if they do?

This paper presented several arguments in favour of substance dualism including:

  1. The distinctiveness of mental and physical properties and states.
  2. The nature of self-presenting properties, as evidenced by private access and incorrigibility.
  3. The subjective nature of experience.
  4. The existence of secondary qualities.
  5. The intentionality, or “aboutness,” of mental states.

Finally, four main criticisms of substance dualism were examined:

  1. The problem of interaction between body and mind.
  2. The problem from evolution.
  3. The problem from Ockham’s razor.
  4. The problem of many minds/counting souls.

Given the above arguments and what I consider to be successfully countered criticisms, I see it as reasonable to conclude that a human being is composed of both mind and matter and that substance dualism is a very convincing position.

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Chalmers, David. The Conscious Mind: In Search of a Fundamental Theory. Oxford: OUP, 1996.

Churchland, Paul. Matter and Consciousness. Cambridge: MIT Press, 1984.

Davies, Paul. The Fifth Miracle: The Search for the Origin and Meaning of Life. New York: Simon & Schuster, 1999.

Graham, George. Philosophy of Mind: An Introduction. 2nd edn. Oxford: Blackwell Publishers Ltd, 1998.

Heil, John. Philosophy of Mind: A Contemporary Introduction. New York: Routledge, 1998.

Hinrichs, Bruce H. ‘The Science of Readings Minds’. Humanist Vol. 61 Issue 3 (2001): 24-27.

Jackson, Frank. Perception. Cambridge: Cambridge University Press, 1977.

Jaquette, Dale. Philosophy of Mind. Eaglewood Cliffs, NJ: Prentice-Hall, 1994.

Maslin, K. T. An Introduction to the Philosophy of Mind. Malden, MA: Polity Press, 2001.

Moran, Laurence. Evolution is a Fact and a Theory. Talk.Origins, 1993, accessed 10 June, 2004; available from; Internet.

Moreland, J. P. and Scott B. Rae. Body & Soul: Human Nature & the Crisis in Ethics. Downers Grove, IL: InterVarsity Press, 2000.

Moreland, J. P. and William Lane Craig. Philosophical Foundations for a Christian Worldview. Downers Grove, IL: InterVarsity Press, 2003), 232.

Robinson, Howard. Matter and Sense. Cambridge: Cambridge University Press, 1982.


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