(I wrote this essay during my A-levels as a revision aid. At the time, my philosophy teacher requested to use it as a revision aid for other members of the class. There were numerous errors in the essay and some explanations weren't necessarily as clear as they could have been. Therefore I've updated the essay and added in any necessary information without re-writing it completely. In other words, I've updated it while trying to stay true to the original where possible. Hence, this may still come across as lacking some serious discussion about the scientific or metaphysical validity of Leibniz's law or the distinction between 'The
Indiscernibility of Identicals' and 'The Identity of Indiscernibles'. Also, proper analysis of Descartes' method and ideas is lacking. The explanation of his ideas that I give leaves out other important aspects of his reason and work - although they are perhaps not strictly relevant here. There is a lack of deeper discussion but for its purposes, this essay is adequate.
Also, the essay does focus most of its criticism on Descartes' Substance Dualism rather than other dualist theories such as that of Plato. This doesn't reflect any dislike for Descartes as a Philosopher. In my opinion, Descartes' Meditations has great historical importance in Philosophy and I regard it as a work of literature which could easily rival other Philosophical eloquence such as Plato's Symposium or Nietzsche's Thus Spoke Zarathustra.)
Dualism
In Philosophy of Mind there are two main types of dualism. These are Property Dualism and Substance Dualism. Property dualism is the theory of mind that while the only 'type' of substance in existence is physical in nature, these physical things can have 'mental' or non-physical properties. I'm not interested in discussing Property Dualism here.
Substance Dualism, on the other hand, is the theory that there are two ontologically distinct substances in existence. There is physical/material substance of the world that we perceive and there's the immaterial/non-physical substance. The latter substance includes minds or souls, beliefs, desires, etc.
When one describes the non-physical 'substance', it's hard to find the correct terms. 'Substance' is a word for physical matter - like such you can place in a test tube, for example. It should be understood that for want of better words, the term 'substance' (or logical substance, to make a Philosophical distinction) has to suffice.
Substance Dualism was famously held by French Philosopher Rene Descartes. Substance Dualism is sometimes referred to as 'Cartesian Dualism' in Descartes honor because he attempted to provide rational arguments for distinguishing the mind and body as separate entities. Descartes Dualism is Interactionist-Substance Dualism. 'Internationist' means that the physical and mental can interact, which isn't a necessary condition of Substance Dualism.
Descartes wasn't the first Substance Dualist Philosopher. The theory is seen in Ancient Greek Philosopher Plato's work 'Phaedo' in which Socrates expresses the Dualist position when discussing death. There's evidence to suggest that Substance Dualism extends back a lot further than Plato and Socrates also. A main impetus for dualism is that it opens up the possibility of a life after bodily death.
The Mental and Physical
The Substance Dualist states that the mental (non-physical) is a substance that is capable of existing independently of the physical. This non-physical substance is empirically undetectable since being detectable requires the physical property of being extended in space.
Below are two lists of qualities. The first list is of qualities that Physical Substance has. The second is a list of qualities that Mental Substance has (according to the Dualist)
Physical
- Empirically detectable
- Are extended in space
- Are subject to physical laws (causality)
- Objective
- Divisible
- Can be doubted
Mental
- Empirically undetectable
- Are not extended in space
- Not subject to physical laws
- Subjective/private
- Not divisible.
- Cannot be doubted
The last point on both lists (in bold) lead us on to Descartes' contribution to the Substance Dualist theory.
Descartes and His Arguments for Substance Dualism
To fully understand and appreciate Descartes' arguments, some background knowledge of his work is necessary. Descartes thought that it was possible that all our beliefs are built on uncertain foundations and therefore are unstable. Descartes saw our beliefs like a building. If the foundations of the building are weak, the building may not stand up. Similarly, if we base our beliefs upon weak foundations, we could have a nasty shock when the foundation gives way and our beliefs all show to be false.
This lead Descartes to hunt for beliefs which you could not reasonably doubt. However, this proved to be an extremely difficult task. Descartes would throw out or suspend judgement on things of which he could conceive the slightest element of doubt. In fact, Descartes supposed that in any given instant, he could be dreaming, insane, or even that an all-powerful, evil demon was deceiving him. The logical possibility of there being such deception prevented Descartes from accepting even the most evident of truths.
It does seem that if he allows for these possibilities, Descartes has effectively denied himself any grasp of any kind of knowledge. However, Descartes finally realised that he did indeed have one piece of knowledge which was beyond all doubt - his own existence. To be dreaming, insane or at the mercy of an evil demon, he must first exist. He is thinking, therefore he exists, or 'I think, therefore I am'.
Descartes noticed that this only proves his existence as a 'thinking thing' and therefore doesn't prove the existence of his physical body. This is important for Descartes first argument.
The argument from doubt is fairly simple. As stated, Descartes could doubt the existence of his physical body but he could not doubt that he exists as a thinking substance. Therefore, they must be different entities entirely. This is based on Leibniz's Law. This
The second argument is the argument from indivisibility. Descartes draws a distinction between the body and mind by claiming that the body is divisible but the mind is not. The body is physical and extended in space, therefore it's divisible. The mind is not extended in space. Descartes gave the example that one could lose a leg and the mind would be unaffected. The body has been divided but the mind remains the same.
Criticisms of Descartes' Arguments and His Use of Leibniz's law
Leibniz's law states that if X has all the properties of Y and also that neither X or Y has a property that the other does not possess, then they are the same thing. This may seem like a truism and this the basis of Descartes' arguments for dualism.
Here is an apparent problem with the law: -
1. Clark Kent is Superman's secret identity; that is, they're the same person (identical).
2. Lois Lane thinks that Clark Kent cannot fly.
3. Lois Lane thinks that Superman Can fly.
4. Therefore Superman has a property that Clark Kent does not possess, namely that Lois thinks one can fly and the other cannot.
5. Therefore Superman is not identical to Clark Kent.
This line of argument is called a reduction ad absurdum. It is supposed to show, in this case, that there are exceptions to Leibniz's law and therefore it isn't valid. There's a more common 'exception' to the law called 'the masked man fallacy': I know who my father is. I don't know who the masked man is. Therefore the masked man cannot be my father.
So does this mean that Leibniz's Law is invalid? Not necessarily. This particular criticism is incorrect. Both the Superman argument and the masked man fallacy make the same mistake. Premise 4 of the Superman argument states that it's a property of Superman that Lois Lane thinks he can fly. Is that really what we mean by a 'property' of Superman? Surely it's better understood that it is a property of Lois Lane that she believes Superman can fly and that Kent cannot.
The argument from doubt commits this very error. Is it a property X that I can doubt it's existence? As seen above, no it's not. So Descartes' doubt of his body isn't a property of his body.
So the argument doesn't necessarily call into question the validity of Leibniz's law. It just shows that we should be very careful with what we regard as properties.
The argument from indivisibility seems flawed. True enough, if one lost a leg, the mind would remain as it was before. However, this is hardly surprising. Most materialists wouldn't claim that the mind resides in the legs. They focus more on the brain. Some types of brain surgery can be conducted without any implications on what we regard as 'the mind'. However, some forms of brain damage cause a person to lose mental abilities that Descartes claim are 'non-physical'. People lose the ability to move or speak. Some lose memories. There are incidents of massive brain trauma which have resulted in very obvious changes in personality and behaviour. This certainly seems like a division of the mind which has resulted from physical division or loss.
