Descartes - Lecture 10
10. Descartes’ Arguments Continued
The Argument from Divisibility
In the Sixth Meditation Descartes argues that as the mind is not divisible whereas any extended thing is always divisible, the mind is neither identical to any physical thing nor dependent for its existence on the existence of any physical thing.
Once again, then, we have an argument that seems to rely on Leibniz’s law. Unfortunately divisible equals ‘can be divided’, which is a modal property and hence one that cannot reliably be used in conjunction with Leibniz’s law. This suggests that the argument needs to be rebuilt around the property not of being divisible but of having component parts.
Let us suppose, however, that speaking of something having a mind is simply a way of saying that this thing has the power of thought. It then seems clear that although it would indeed be absurd to talk of something having part of a mind, i.e. part of the power of thought, this absurdity would nevertheless not have any tendency to show that something could have a mind/the power of thought without being an extended thing of many parts.
The Problem of the Interaction between Mind and Body
Elementary instances of causal interaction between mind and body – someone’s decision to move their hand causes their hand to move; light falling on the retina causes one to acquire the belief that one’s hand is approaching the desired mug of coffee.
Not all mental causes and effects are things that possess propositional content like beliefs and decisions. There are also such mental phenomena as pains, tickles and itches, passions, and the phenomenological contents of sensory experiences.
Descartes’ famous acknowledgement of the complexity and intimacy of the relationship between mind and body ‘I am not merely present in my body as a sailor is present in my ship, I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit’ (Sixth Meditation). Whereas a sailor might simply acquire the belief that the ship’s mast has been broken, we do not, in normal circumstances, merely acquire the belief that our arm has just been broken. Rather we also experience agonising pain.
How much sense, though, can we make of the supposition that two things as radically different as a physical object and an immaterial mind might causally interact? Descartes thought of physical objects as exercising their causal power in relation to other physical objects through pushing and pulling and surface channelling. Today we might think instead of gravitational attraction, electromagnetic attraction, strong nuclear interaction (forces between nucleons in the atomic nucleus), weak nuclear interaction (an interaction between elementary particles that is responsible for certain decay processes). In neither case, though, are we likely to happy that we have any understanding how such mechanisms could have any causal influence on non-extended immaterial substances.
It might be suggested in response to this difficulty that we should abandon the supposition that genuine awareness of something as a instance of causal interaction must involve an understanding of how the effect must necessarily come about given the nature of the event picked out as the cause. A commitment to this supposition manifests itself in Descartes’ own writings in such principles as the need for there to be at least as much reality in the cause as in the effect and the requirement that the cause should resemble the effect. Thus abandoning this supposition would undermine large sections of Descartes’ overall metaphysical position. If we were willing to pay this price, however, one metaphysically minimalist account of causation that could be pressed into service here is the regularity account often associated with the philosopher David Hume. This account claims that an event x is the cause of an event y if y occurs after x and events of type x are regularly followed by events of type y. Thus no a priori restriction is apparently placed on what kind of thing might bring about any other kind of thing. On the other hand, it might still be suggested that even this regularity account needs to be supplemented by some temporal and spatial restrictions. Every time the space shuttle is launched, this event is no doubt followed within a short time by a star exploding somewhere amongst the universe’s myriad galaxies. However even a firm supporter of the regularity account of causation would presumably wish to deny that launching the space shuttle causes stars to explode. Thus it is tempting to supplement the regularity account with a clause to the effect that x is a cause of y only if events similar to x are regularly followed by events similar to y that occur either near to these x-like events or bear some systematic spatial relationship to such events. Unfortunately Descartes is, of course, insistent that minds do not have any spatial relationships with physical objects. Thus it seems that even a pure regularity of causation has difficulty making sense of the supposition that events in a Cartesian mind might interact causally with physical events.
Moreover even if this conceptual difficulty can be resolved, can we square the supposition that immaterial minds exert causal influence on physical phenomena with our knowledge of neuro-physiology? Our neuro-physiological theories are by no means complete at the moment, but it might be thought that they are already sufficiently developed that it is plausible to suppose that they will ultimately be able to justify us in holding that all our bodily movements be accounted for as the results of prior, purely physical causes. What scope, then, is there for thinking of immaterial causes as playing any role in the process by which our actions come about?
Other Problems for Cartesian Dualism
Distinguishing between numerical and qualitative identity
If minds can be non-physical substances, it should be possible for us to give some account of what would make one non-physical mind separate from another mind. Let us suppose, however, that two people, A and B, are simultaneously thinking about Hellenistic scepticism. Furthermore they have, for the past ten seconds, been following exactly parallel chains of thought. What would entitle us to say that we have in this situation two substances rather than one? When we are concerned with qualitatively identical physical objects existing at the same time, we can distinguish between them according to their respective spatial locations. Descartes, however, would insist that mental substances never possess a spatial location. How, then, could he assist us to make sense of the supposition that mental substances are individual entities?
Evolution
On the dualist view there is an absolutely sharp distinction between being a merely physical entity and having both a physical body and an immaterial mind. However evolution doesn’t generate such radical discontinuities. The first biological organisms to emerge on this planet presumably didn’t come complete with non-physical minds. However we are, according to Descartes, the possessors of such minds. Thus it seems that unless the dualist is prepared to reject the theory of natural selection and anything analogous to it out of hand, he will be forced to concede that on the material front there is a gradual development of organic complexity, while insisting that on the mental front there is at some point a radical jump between creatures without non-physical minds and those with them. Obviously the supposition of such a discontinuity is not a self-contradictory supposition, but there doesn’t seem to be any principled way of explaining the existence and location of the alleged discontinuity.
Other minds
Even if Descartes is confident that he has a non-physical mind, why should he claim as much about other human beings? After all he is not aware of their minds in the way in which he is aware of his own, and to argue that they must have non-physical minds just because he is aware of one case, his own, where a human body is associated with a non-physical mind seems a ludicrously rash generalization on the basis of woefully insufficient evidence
Descartes’ own answer is that if any purely physical systems bore a resemblance to our bodies and imitated our actions as closely as possible for all practical purposes, we would still be able to distinguish them from Cartesian people because they would never be able to use language with the fertility and appropriateness we do and their other problem-solving abilities would inevitably have a narrower range of application than our own abilities.
Unfortunately this answer seems to take an excessively dogmatic view of the capacities of purely physical systems. Current computers can already mimic a surprising variety of human behaviours and responses with sufficient fidelity and flexibility to deceive unwary human observers into believing that they are dealing with another human being. Yet a human brain is a vastly more complicated physical system than any computer yet built. So it seems very rash to maintain that we know that a physical system of that level of complexity would not be able to do all we can do unless it was augmented by a non-physical Cartesian mind. After all, even if it is true that no purely physical system would be able to respond in an appropriate way to all physically realizable circumstances, there is no good evidence to support the conclusion that we possess such a capacity either.
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