Substance Dualism is the view that the mind and body are distinct. It pictures the world as consisting of two independent domains, the mental and the material, each with its own distinctive defining properties. We have clear and distinct awareness that the body is physical therefore it is spatially temporary and made from atoms which follow physical laws. It has dimensions, mass, texture etc. We also have clear and distinct awareness that the mind is non-physical therefore is not made from atoms or follows physical laws. The essence of the body is to be extended whilst the essence of the mind is to think. The mind is logically distinct and an independent entity to the body. So what can be asserted for one cant be asserted for the other. The mind will continue to exist without the body. It is infinite and immortal and will go to the afterlife. But the body is subject to decay because it is physical. It is not immortal and cant exist without the mind. The claim is there are two completely distinct substances, where substances mean a fundamental thing rather than stuff.
In Descartes Sixth Meditation, he states one of his arguments for the mind and body being distinct. This is known as The Conceivability Argument:
1. I can clearly conceive existing without my body
2. What is clearly conceivable is possible
3. So it is possible for me to exist without my body
4. So I am neither identical with, nor a part of my body
5. So substance dualism is true
Descartes, when considering himself, had a clear and distinct idea of myself (himself) inasmuch as I am (he is) only a thinking and unextended thing. Since he is thinking now, he must be a thinking thing. This property must be there as it belongs to his essence. If thinking ceases then he doesnt exist, an argument which he tries to prove in his early works in The Discourse. He has a clear understanding of what thought is. This allows him possibly to be non-extended as spatial temporality is not essential to him. He also has a distinct idea of body, inasmuch as it is only an extended and unthinking thing. If we think about an object we comprehend that its necessary extended and its essence is not thinking. For example, the essence of a stone is to be extended not to be a thinking thing. It is very hard to comprehend a stone thinking and if we can it is definitely not an essence of the stone.
It is possible to conceive of the mind existing without the body and the body existing without the mind. Descartes uses the word possible in the sense that it is logically conceptual therefore not self contradictory that he can exist without his body. This is because it is possible to conceive of something being extended and non-thinking. Also it is possible to conceive of something being non-extended and being a thinking thing. For example we can conceive of my mind thinking but having no spatially temporality. God can create a world where whatever is conceivable is metaphysically possible because all things which I apprehend clearly and distinctly can be created by God. Thus anything conceivable becomes an actual truth. For example God cant create a square circle as it is not metaphysically possible. But God can create me, a thinking thing, distinct from my extended body since they may be made to exist in separation at least by the omnipotence of God. Therefore it is possible that my mind can exist without my body resulting in having a body not being essential to me.
Descartes is moving from this doubt of his body to his essence. Descartes is using his scepticism that he is not going to accept anything that he is not certain of. He is certain of his own thinking and existence though. He concludes that this he can be sure of but this is the only thing he can be sure of: I am a real thinking thing and really exist, but what thing? I have an answer: A thing which thinks. But there are two different meanings to a thinking thing. One is something that thinks or another is something whose essence is to think, a thing that cannot not think. These are very different claims. He is making an epistemological claim to a metaphysical claim. He is arguing from how we come to know something to what it actually is. The argument shifts from a possible sense to a possible fact: It is possible that I am a thinking thing to my essence is a thinking thing. Knowing that I am thinking, it does follow that I am thinking in one sense. If I am thinking then I must be capable of thinking. But the claim that his essence is thinking is fallacious. If we take John Lockes claim that God can make matter think then God can make a stone think. A stone would then know that it is only a thinking thing and not know that it is extended. Therefore as its only aware of its own perceptions it can conclude that its essence is thinking. Just as God has made the stone think, God can take this away. Therefore the stone is left with its essence being extended and non-thinking.
Moreover, all because the mind and body have different essences, this does not mean that they have to be two different substances. Surely they can be one and the same thing. Cant there be one entity which is thinking and extended? Jerome A. Shaffer uses the example of one entity, a man:
The defying characteristic of being a husband is being a married man and the essence of being a parent is having offspring, but one and the same person can be both.
Hence all because we have conceived of two entities having different properties it does not mean that they actually exist distinct from each other.
For Descartes it would seem that they have completely distinct essences that it is impossible for them to not be distinct. Therefore the example of a husband and a father is seemingly weak because the actual properties of the occupier of the essences are both extended and male. Whereas the properties of extended and thinking things have, according to Descartes nothing in common. It would appear that there needs to be a stronger example from Shaffer to show that the mind and body can be one and the same thing.
George Graham has a stronger example of the Gods making Oedipus marry his mother. As he didnt want to marry her he would say: very idea disgusts me, I find it unconceivable that I will marry my mother. Oedipus then falls in love with Jocasta, who is actually his mother. If we asked him what he thought about marrying her he may say: Very idea pleases me, I find it very conceivable. We would then conclude that Oedipus can conceive of himself marrying Jocasta but cant conceive of himself marrying his mother. Therefore they are not the same person. If I can conceive of myself as disembodied but cant conceive of myself without a mind you cant conclude that your mind and body are distinct as they can be one and the same thing. This is because the conclusion is aspectual and realises on aspectual knowledge.
This aspectual knowledge that Graham mentions is a major downfall to Descartes conceivable argument. Gottlob Ferge explains this with the ancients knowing that Venus is the morning star whilst believing that the evening star was Venus as well or illiterate people not knowing that water is identical to H20. The mind and body could be the same thing but he they are assessed differently, one by introspection and one by inspection but this doesnt mean they can be separated and exist apart from each other.
W. D. Hart creates a story to strengthen Substance Dualism and tries to show that the mind and body are not one and the same thing. This is because we can conceive the mind to exist without a body: Imagine one morning you wake up and go to the mirror. Looking in the mirror you realise that your eye sockets are empty. You can visualise your face with empty eye sockets as it would look to you in the mirror. You then saw around your head and see that your brain is not there. You can visualise how your empty brain pan would look to you in the mirror. Lastly it is still possible to visualise if you had no body. You would see the reflection of the room in the mirror. So you have a recipe for visual experience of yourself disembodied. As this is a thought experiment it could not actually occur in the real world. But the fact that we can imagine being disembodied. As we can imagine being disembodied without contradiction, it means that it is metaphysically possible. Hart would conclude that it is metaphysically possible for me to exist disembodied. Therefore the mind and body are distinct as disembodied existence can occur.
Is it not possible to conceive of water not being identical to H2O even though it is metaphysically impossible? If we can then it is possible to conceive of the mind existing apart from the body doesnt show that it is metaphysically possible. For this metaphysically possible for the mind to exist with the body then the mind must be numerically distinct from the body. Kripke used his concept of rigid and non-rigid designators for Cartesian Dualism argument to prove that they are numerical distinct:
If a and b are rigid designators, it follows that a = b if true, is a necessary truth. If a and b are not rigid designators, no such conclusion follows about the statement a = b
The rigid designator water is identical to the rigid designator H2O. This is true scientifically. Water is identical to H20 in every possible world. If we have two rigid designators and the two are identical then this will mean that conceivability entails metaphysically possibility. This is because we cant conceive of a possible world where H2O is not Water. According to Descartes this would mean that God couldnt create a world where Water is not H2O as they are the same substance with the same essences. We cant conceive metaphysically that water is not H2O as this would make no sense.
However it does not work if we use water non-rigidly. Water may not be a direct reference to H2O but a summary of the properties. This would be topic neutral as we are leaving out the nature of water. We could have a sample of water but not know that it is H2O, just like our predecessors. We cant conceive of them being identical in every metaphysically possible world therefore H2O is not water. We can easily conceive a non-rigid designation of water, for example tap fluid, rain, a lake that may not have been H2O as it is only there empirical properties that we know. As long as it is a non-rigid designator of water and we produce the properties, water can be anything. Thus we could conceive of the substance described as having a different chemical composition in a possible world.
Kripke applies this to a brain state being identical to a mental state and tries to improve Descartes argument. We are immediately subjectively aware of mental states. We have immediate perceptions of them inside me. What you are experiencing when in pain is pain. It is an internal content of consciousness and this is the only way we can understand it. Therefore it is a rigid designator. If we can conceive of mental states being identical to the body then this would mean that it would be a necessary truth. If Descartes can conceive of himself being distinct from his body then Descartes is not identical to his body.
Through modern neuroscience research it is possible to see that Descartes immaterial explanation of thinking being the essence of the mind is a lot simpler than getting a brain dependent theory. He believed that:
Each substance is thought to have its own laws and its own range of properties; hence research on the brain is not going to yield any knowledge of the mind.
But as more research occurs with neuroscience and neurology the more we realise how absurd the concept of Substance Dualism is. The mind apparently has higher functions which are reasoning, consciousness, emotion etc. All of these higher functions can be changed by drugs or an injury. For example damage to the temporal Lobe, the person with this damage will learn complex cognitive skills but will be unaware he has done so, even while engaging in them. There are many other examples of how the brain affects our higher functions. If the brain is dependent on the mind then how can this be true? How can the brain affect thinking as thinking is not its essence? Therefore it would seem that dualism fails not so much because of our limited knowledge of the mind but rather because of modern science. As the conceivable argument needs the essence of the mind and body to show that they we can conceive of them distinctly, if Descartes has got the essences wrong then he cant conceive of himself disembodied.
Doesnt entail it false as further research may discover there is a substance dualism but at the moment there is no leads.