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.
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 is some point in space 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.
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 possible 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 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
Daniel Dennett argues that anyone who thinks they are imagining a mind without a body is confused. He argues it’s like trying to conceive of health separate from the functions of the body. It can be imagined, but only by someone who thought health is something ‘additional’ to the proper functions and workings of the parts of the body. Dennett claims that while that would be an obvious mistake in the case of health, the case of consciousness is no different. Conceiving of consciousness removed from the body while leaving all cognitive systems intact is an ‘entirely bogus feat of imagination’ dependent on imagining consciousness to be some sort of consciousness-module that might or might not be present in the body.
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 is actually not the true mind, but a concept of the mind based on ignorance.
Integration: The conceivability argument begs the question. When claiming we can conceive of the mind without the body, that assumes that the mind is not the body. If physicalism is true and the mind is the body, then our conception of the mind without the body must be confused. So, we can only conceive of the mind without the body
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 we know what the mind is. He assumes that the way the mind appears to itself is how it really is, such that if it appears non-extended, then it is non-extended and thus cannot be an extended thing like the body. However, perhaps the mind appears non-extended to itself while in fact being identical to the brain.
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.