Descartes on the Replacement Argument?

(You can read the context here, but I’m taking the translation from the IEP)

[T]here is a great difference between the mind and the body, inasmuch as the body is by its very nature always divisible, while the mind is utterly indivisible. For when I consider the mind, or myself in so far as I am merely a thinking thing, I am unable to distinguish any parts within myself; I understand myself to be something quite single and complete….By contrast, there is no corporeal or extended thing that I can think of which in my thought I cannot easily divide into parts; and this very fact makes me understand that it is divisible. This one argument would be enough to show me that the mind is completely different from the body…. (AT VII 86-87: CSM II 59).

The IEP sets up his argument in the following way:

1.I understand the mind to be indivisible by its very nature.
2.I understand body to be divisible by its very nature.
3.Therefore, the mind is completely different from the body.

It doesn’t take much to realize this argument is missing something and is overstating its case. The conclusion does not follow without the following hidden premise:

(HP) If the mind and body have different properties, then they are completely different.

However, I don’t see that premise as true at all. In fact, it is extremely counter-intuitive. But we can mend the argument in a way Descartes would probably object to:

1. The mind has the property of being essentially indivisible.
2. The body is has the property of being essential divisible.
3. If the mind and body are identical, they have the same essential properties.[The law of indescernibility of identicals: x = y → (∀F)(Fx ↔ Fy)]
4. The mind and body do not have the same essential properties [from 1,2]
.: The mind and body are not identical [MT from 3,4]

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  1. […] Descartes on the Replacement Argument? […]

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