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Cartesian dualism Cartesian dualism is the claim (CD) Man is a compound of two distinct substances: unextended thinking substance (mind) and extended, unthinking corporeal substance (body). The two types of substance are mutually exclusive. Distinguish three claims: the real distinction, substance dualism and property dualism: (RD) Mind and body could each exist apart and are therefore really distinct substances. (SD)
There are essentially two kinds of matter. Any substance with material properties lacks mental properties, and any substance with mental properties lacks material properties.
Mental properties and material properties are mutually exclusive.
Note that one could accept (PD) but reject (SD): one could allow that a single substance had properties of two distinctively different sorts. We can see Descartes as arguing from (SD) and (PD) to (RD). Descartes uses the word soul in a far wider sense than we would today. For Descartes, 'soul', 'mind' and 'res cogitans' ('that which thinks') are all synonymous, and all include a wide range of conscious mental activity. In the 17th century the 'soul' need not be taken as immaterial and purely spiritual. As J Cottingham points out, according to the Aristotelian account to say the 'body' is 'animated' is to sat the matter of the body is organised in such a way that it has certain capacities (thinking, digesting, moving,...). On the standard version of this account, the soul must be embodied in matter to produce a functioning human being. On the other hand, there were also crude physicalist accounts - taking the soul as 'something...like a wind or fire' permeating the solid body - available in Descartes' time. Against these kinds of view, Descartes holds the soul to be essentially non-material, a separate, separable spirit. As Descartes puts it in the Comments on a Certain Broadsheet, men are something we regard as having at the same time thought and extension; they are "composite entities", an entity consisting of a soul and a body. According to Descartes' metaphysics, as Hatfield puts it, sense perception and imagination depend for their existence on the mind-body union. There can be intellectual perceptions that do not depend on the brain, but acts of imagination and sense perception require the brain. On the other hand, according to Hatfield, in works published posthumously Descartes developed an extensive physiological description of animal bodies, in which he explained the functions of life in a purely mechanical manner, without appeal to a soul or vital principle. He mechanised the notion of a living thing, and in doing so equated soul with mind. He regarded 1
nonhuman animals as machines, devoid of mind and consciousness, and hence lacking in sentience.
Nature of substance Descartes takes a substance to be something capable of independent existence, as depending on no other thing for its existence. In the Principles, he says that 'substance' applies univocally to mind and body, and that to each substance there belongs one principal attribute. For mind this is thought, and for body it is extension.
Why does Descartes hold this view?
Margaret Wilson puts the view down to the combination of the fact that Descartes subscribed to a universalist, mechanistic physics, but did not believe human intelligence could be accounted for mechanistically. He felt this was due to the inability of mechanistic models to respond to a range of novel stimuli, and to account for language (a point Chomsky drew on). Thus, the brain cannot be involved in pure understanding - the activity of ratiocination, of logical thought - because there is no role for it to perform. Instead, our ability to engage in intellection must be due to something wholly incorporeal.
A 'real distinction' In the Principles, Descartes distinguishes between three types of distinction: 'real' (between substances), 'modal' (of modes), and 'conceptual' (between a substance and its attributes). In I.60, he says that "we can perceive that two substances are really distinct simply from the fact that we can clearly and distinctly understand one apart from the other". God can bring about anything of which we have a distinct understanding. Thus, since we have a distinct understanding of ourselves as a thinking thing, and can exclude every other thinking or corporeal thing from ourselves in our thought, it follows that we are really distinct from every other thinking substance, and every corporeal substance. Descartes presents a criterion for real distinctness: being separable by God. So even if God were to compound two substances into an extremely tight unity, so long as He has the power to separate them, they are really distinct.
Descartes' three arguments for dualism Hatfield argues that Descartes' argument takes the form of a metaphysical insight into mind and body. We can see the entirety of the Meditations as building to this conclusion: Med. II: prepares for thinking of mind as independent of body, via clear and distinct perception of mind without any concept of body. Med. III-V: shows that clear-and-distinct perception yields truth. 2
Med. V: argues for clear-and-distinct perception of the essence of body. Med. VI: argues that mind and body are substances with mutually exclusive essences, and hence really distinct.
The argument from doubt Descartes presents an argument on the following lines in Part Four of the Discourse, and also makes a similar argument in Meditation II:
I could doubt that my body exists. I could not doubt that I exist. Therefore, I am distinct from my body. Therefore, I could exist even if my body did not.
For example, in the Discourse he writes "while I could pretend that I had no body...I could not for all that pretend I did not exist...accordingly this 'I' - that is, the soul by which I am what I am - is entirely distinct from my body...and would not fail to be whatever it is, even if the body did not exist".
Arnauld's objection But, as many philosophers - going back to Antoine Arnauld in the Fourth Objections and Replies - have noted, the argument is not valid. Arnauld's objection is that he can doubt whether a right-angled triangle satisfies Pythagoras' Theorem, but certainly does not imply meeting this property is not an essential to being a right-angled triangle. In the Synopsis the Meditations, Descartes seems to accept the force of this criticism, and row back from the argument. Cottingham points to Descartes' comments on the Second Meditation, published in 1647, in which he writes "when I said that the soul knows itself as an immaterial substance ...in spite of this we are not sure there is nothing material in the soul". To take Hatfield's analogous explanation of why the argument is fallacious:
1. I cannot doubt the existence of a masked man, Zorro.
2. I can doubt the existence of Don Diego (he may have died, for example).
3. Therefore, Zorro is not Don Diego. In the Synopsis, Descartes says that in Mediation II the meditator rules out that the concepts of mind and body are distinct - but cannot rule out their possible identity. It is only after the introduction of the truth rule and hence the claim that clear and distinct perception is sufficient for truth that Descartes can argue that, since clear and distinct perception accurately portrays the distinction between mind and body, they are really distinct.
The argument from clear and distinct perception In the Sixth Meditation, Descartes turns to attempting an explicit proof of his dualist thesis. This argument takes the form
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