Cartesian Dualism

• Substance = an entity that does not depend on another entity to exist.
• Substance dualism: holds that there are two types of substance, mental (eg. the mind) and material (eg. the body). 
• Substance dualism can be contrasted with materialism, a type of monism which asserts that there is only one kind of substance, which is material.
• Descartes, a substance dualist, held that the mind, as a separate substance with distinct properties, could exist entirely independently of the body.

• Leibniz's Law is an ontological principle which outlines the requirements necessary for two things to be considered identical: If A is identical with B, then any property of A is a property of B.
• Properties which must be shared include spacio-temporal location. If two objects are identical in all respects but this, then they are only qualitatively identical - eg. two billiard balls from the same production line.
• George Orwell and Eric Arthur Blair are numerically identical: they share all properties, including spacio-temporal location (so they are one and the same thing). It is this form of identity with which Leibniz's Law is concerned.

• The simplest argument for dualism is based on Leibniz's principle of identity.
• It begins by considering the separate properties of mind and body.

Mind: Exists in time, but not in space; indivisible; private; cannot be in error about mental states; can reason. 

Body: Exists in time and space; divisible; public; can be in error about the physical world; cannot reason.

• If these differences are valid, a deductive argument can be derived from the application of Leibniz's Law:

1. If mind is identical to matter (the body), then mind must have all the same properties as matter.

2. Mind has different properties to matter.

3. Therefore, mind is not identical to matter.

(4. And therefore, my mind is not by body.)

• Various deductive arguments can be derived from the different properties of mind and body.
• For example, Descartes says that having parts is an essential property of bodies. Bodies exist in space, and can therefore be divided. The essential property of minds, he claims, is thought. 
• So: The body is divisible; the mind is indivisible; therefore the mind is not the body.
• But we can object that his argument regarding indivisibility (for example) is not sound.
• This is because we can object to the truth of the second premise: 'the mind is indivisible'.
• Consider mental illnesses like multiple personality disorder... In these cases, it seems that the mind is divided into parts, in the sense that not all parts of the mind are able to communicate with one another.
• Similarly, the Freudian model of consciousness implies that we have both conscious and subconscious elements in our mind.

Response:  Descartes could respond that the way in which the mind is divisible is completely different to the way in which the body is. You can literally physically divide the body, but the mind cannot be physically broken or divided.

• The most important of Descartes' arguments is his 'Argument from doubt'.
• In his 'Meditations', Descartes questions what kind of thing he is. The question 'what am I' can be answered by considering the question of what it means for 'I' to exist... 

Cartesian Doubt:

• Descartes first remarks that he can coherently doubt that he has a body - he only believes he has a body as a result of fallible personal perceptions.
• He also considers that perhaps he is being deceived completely by an evil demon.
• But he then asserts that he cannot doubt that he has a mind - if the demon were to make him doubt that he is thinking, then the very act of doubt would prove that he is thinking.
• This in turn proves that he exists, as a thinking thing.
• The conclusion: cogito ergo sum - I think therefore I am.

Descartes concludes that it is logically possible for his mind to exist without his body, because it is fully conceivable.

The Argument from Doubt can be formulated as follows:

1. I can doubt that I have a body

2. I cannot doubt that I have a mind

3. Therefore, my mind is not my body. 

4. Therefore I am not my body.

• Again, Descartes' is drawing on Leibniz's Law.
• If the mind and body have the opposing properties of divisibility and indivisibility, they cannot be identical: so they are not the same substance.

• Descartes claims that he is a thinking substance. Philosophers often feel that he is trying to show that he is the same 'I' enduring through time. 
• But how can Descartes actually be sure of this? Couldn't he just be a succession of thoughts?
• AJ Ayer framed this argument as follows:
• In order to follow through with his method of doubt, Descartes cannot attach an 'I' to his thoughts with any certainty. 
• For example, when we say 'it's raining', there is no 'it' which is raining... We're just subscribing to a grammatical law of subject and predicate.
• So, when I say I think, I am simply being tricked by the laws of grammar, by adding a predicate.
• The only thing Descartes can be certain of is 'there are thoughts'

Descartes responds that thoughts logically require a thinker. Properties cannot exist without substances; thoughts are, logically, properties of the mind. But again, this seems to be nothing more than an assumption - perhaps thoughts are substances in themselves, and consequently are able to exist independently of any other substances (namely, a thinker).