Descartes’ Substance Dualism

Descartes’ substance dualism is the theory that there are two distinct substances, mental and physical. A substance is a fundamental type of existence which can’t be broken down into anything else. The essence of mental substance is thinking, the essence of physical substance is extension. A thing is extended if it takes up physical space, is located, has coordinates.

The indivisibility argument

Descartes argued that the essential property of physical substance is extension. Anything that is extended in space can be divided, because there has to be some point along which it could conceivably be divided. The mind does not appear to be divisible, however, because it seems to be non-extended. The mind is not located in space; it does not have spatial coordinates along which it could be divided.

Leibniz’ law is that identical things must have the same properties. The physical has the property of being divisible but the mental does not. If the body and mind were identical, then that one identical thing would be both divisible and indivisible, which is impossible. Therefore, the mind and body are not identical.

P1. Physical substance is divisible (since it’s extended).
P2. The mind is indivisible (since it’s non-extended).
P3. Leibniz’ law is that identical things must have the same properties.
C1. The mind therefore cannot be identical with any physical substance, such as the body.

The issue that the mental is divisible

The mental is divisible. The mind can be divided into perception, memory, emotions and so on. So, the mind and body share the same property of divisibility and thus could be identical.

Integration: This attacks P2, the premise that the mind is indivisible, in which case the mind does have the same properties as the physical body, making the indivisibility argument false.

Descartes responded that by the mind he means consciousness. Perception, memory and feeling are not divisions of consciousness, they are different modes of consciousness. It is the same undivided mind that perceives, remembers or feels emotions.

Divided hemispheres. Discoveries in modern science can be used to push the objection further and tackle Descartes’ account of the mind as consciousness head on. There are phenomena which have been discovered by modern psychology which could suggest that actual divisions of the mind are possible, e.g. blindsight & separated brain hemispheres. The brain is split into two hemispheres which are only connected by a thin strand of neurons called the corpus collosum, which can sometimes be cut as a medical procedure to treat epilepsy. This has the bizarre effect of, some have argued, seeming to result in two separate ‘persons’ in the one body. The right hemisphere controls the left arm and the left hemisphere the right arm. Patients have been observed picking up some food with one arm and the other arm hitting it away. One case involved a patient who tried to hug his wife with one arm and push her away with the other.

Integration: This is at least good evidence for the possibility of consciousness being divisible and gives us reason to think P2 false and the indivisibility argument therefore fails.

It also casts doubt on Descartes’ method in assuming that we have the full picture of what our mind is merely from how it seems to be to itself, which is his justification for P2.

The issue that not everything thought of as physical is divisible

Quarks are what neutrons and protons are made up of. Quarks are elementary particles, meaning they are not made up of smaller particles. They are fundamental; they cannot be broken down into anything smaller. This means they are indivisible. Descartes could not have known about Quarks as they had not been discovered in his time.

Integration: This shows that P1 is false, since quarks are physical yet indivisible. Without P1, even if Descartes is correct that the mind is indivisible, it could be an indivisible yet physical thing.

Descartes would respond that he rejected the theory of atomism; that objects are made up from indivisible particles. Anything extended in space takes up a finite amount of space. We can conceive of it divided at some point in the area it is extended in. God would have the power to divide it. It follows that nothing can be both extended and indivisible.

However, according to some interpretations of quantum mechanics, particles are not thought of as extended in space with precise coordinates. In fact, particles are thought to not have exact positions and velocities, but a probability distribution about where they could be and their movement. Empiricism has shown that physical matter is far stranger than was suggested to Descartes by his intellectual clear and distinct perception of it.

Unusual evaluation development issue: If both issues against Descartes’ indivisibility argument succeed, then his argument succeeds! Since then, the mind would be divisible yet the physical is not necessarily divisible. In that case, the mind and body/physical do not share all their properties and therefore cannot be identical.

The conceivability argument

P1. I have a clear and distinct idea of myself as a thinking non-extended thing.
P2. I have a clear and distinct idea of my body as a non-thinking extended thing.
C1. These opposing properties allow us to conceive of the mind separate to and without the body.
P3. What is conceivably separate is possibly separate.
P4. What is possibly separate is actually non-identical.
C2. Therefore, the mind and body are not identical.

Since the mind is non-extended, it is possible to conceive of it without any extended thing, existing independently of anything physical. For example, you could imagine being an immaterial ghost walking through walls.

If two things are actually identical, then they cannot possibly be separate. E.g., a triangle cannot possibly be sperate from having three sides. The separability of two identical things is not even conceivable. E.g., we cannot conceive of a triangle without three sides.

So, since we can conceive of the mind without the body it follows that it is possible for the mind to be separate from the body, from which it follows the mind is actually not identical to the body.

The issue that the mind without the body is not conceivable

If physicalism is true, then there is some scientific explanation for how the mind is physical, e.g. is identical to certain brain processes. We might one day discover that.

So, if physicalism were true, anyone who conceives of the mind without the body would just be confused, lacking understanding about how the mind is the body.

This could be the situation we are in when following Descartes’ reasoning and trying to conceive of the mind without the body. If physicalism is true, such a conception is confused. In that case, the validity of the conception presupposes that physicalism is false.

If Physicalism is true, the mind can only be conceived separate from the body by someone ignorant of the nature of its connection to the body. E.g imagining yourself as a ghost floating through walls not interacting with the physical. What is being conceived of there, for all we know, is actually not the actual mind, but a concept of the mind based on ignorance.

Integration: The premise that we can conceive of the mind without the body is only valid if we assume physicalism to be false. This argument attempts to prove physicalism false, but has to assume physicalism is false to do that. So, Descartes’ argument, specifically C1, begs the question.

Descartes would respond that he gives us reasons to think we can conceive of the mind without the body, however. They have opposing properties – non-thinking/thinking and non-extended/extended.

However, Descartes assumes that the way the mind appears to the mind is how the mind actually is, such that if it appears non-extended, then it is non-extended and cannot be an extended thing like the body. He thinks that a clear and distinct intellectual perception of the mind shows us what it really is; a mental substance with the essential property of thinking. However, perhaps the mind appears clearly and distinctly non-extended to itself while nonetheless in fact being identical to the body/brain. The problem for Descartes is that he thinks he knows what the mind is, and on that basis claims that his conceiving of it as separate to the body reflects reality.

An empiricist would object to this assumption. Until we have discovered what consciousness actually is, we cannot decide that what people claim to imagine it to be really is consciousness. This seems to cast doubt on the validity of any conceivability argument.

The issue that what is conceivable may not be possible

Masked man fallacy. Arguments reliant on inferring possibility from conceivability are susceptible to the masked man fallacy. Imagine someone heard about a masked man robbing a bank. They can conceive that it’s not their father, but if it really was their father then it’s impossible for it to not be their father, yet that was what they conceived. Therefore, we can conceive of the impossible. In that case, something’s being conceivable does not mean that it is possible.

Integration: this attacks P3, that what is conceivable is possible, by showing that we can conceive of the impossible.

The masked man fallacy only shows we can conceive of the impossible due to ignorance. The person in the example is ignorant of who is under the mark which is what allows them to conceive of the impossible. However, Descartes would argue that there is no analogous ignorance when it comes to our knowledge of our own mind. In the case of our mind, once properly understood through clear and distinct intellectual perception, there remains no ignorance that could lead to the conceiving of something impossible as happens in the masked bank robber case.

However, again Descartes assumes that the way the mind appears to the mind is how the mind actually is. He thinks that a clear and distinct intellectual perception of the mind shows us what it really is; a mental substance with the essential property of thinking.

Nonetheless, evidence from neuroscience and psychology shows us that we are ignorant about our mind in many respects. For example, most mental processing is unconscious and we are consciously unaware of the way in which our mind is influenced by it. This provides strong evidence against the view that the mind’s perception of itself is perfectly accurate let alone complete. So, it’s possible that we are ignorant about our minds and for all we know from out self-perception, our mind is extended and identical to our body. It follows that when we conceive of our minds as non-extended and thus separable from our body, it is possible that we are conceiving of the impossible due to our ignorance that our mind is our body.

Integration: So again, this defence against the relevance of the masked man fallacy begs the question by assuming that we know our mind is non-extended in order to prove that the mind is non-extended.

The issue that what is metaphysically possible tells us nothing about the actual world

Integration: Even if Descartes’ argument could show that the mind being separate from the body is conceivable and possible, we don’t have to accept that it is actually separate.

For example, water is identical to H2O, but it seems that we can conceive of water not being H2O, especially before we discovered that it is. So, following Descartes’ logic, it seems conceivable for water to not be H2O, so there could possibly have been a world where water and H2O are separate. Yet, in the actual world water is identical to H2O. So even if we accept conceivability and possibility of separability, we don’t have to accept actual separability.

All Descartes has shown is that it is within the metaphysically possible states of affairs for a mind to be separate to the body. That doesn’t show that it is actually separate, nor does it even tell us whether it is actually separate or not. The actual world that exists is only one of an immense number of metaphysically possible worlds. In some of them physicalism could be true and in others substance dualism could be true.

However, this response fails to understand identity. In the case of water, it is identical to H2O. Kripke’s theory of ridged designators claims that identity relations must hold true in all possible worlds. Otherwise, we get the absurd outcome that it’s possible for something to be what it is not. Water is H2O, so there is no possibility of it not being H2O, otherwise something could possibly not be what it is, which is absurd. Our being able to conceive of water not being H2O only shows that it is logically possible for water to not be H2O. Yet, because it is the case metaphysically that water is H2O, it is metaphysically impossible for it to not be.

The physicalist claim is something like that the mind is identical to the brain, or to certain brain processes. In that case, on the theory of ridged designators, if the mind is the brain, then it can’t not be the brain. If the mind is the brain, then there can’t be a metaphysical possibility of it being a cartesian mental substance.

Integration: Physicalism makes an identity claim and as such cannot restrict its claim to what is true in the actual world. It is committed to the identity claim holding in all possible worlds. In that case, a physicalist must deny either the premise that the mind is conceivable (C1) or that what is conceivable is possible (P3). It cannot hold that the mind and body are possibly separate but then deny that they are actually separate.