2. Cartesian Dualism
A philosophical (or other) theory is called dualist or a dualism if it posits two different kinds of fundamental things, neither of which can be reduced to or explained in terms of the other or anything else. In this section, we’ll look at Substance Dualism, the most elaborate development of which has been put forward by Descartes. It is also called Cartesian Dualism. Its basic tenet is that there are mental as well as physical things.
Dualism can be contrasted with monism, according to which there is only one fundamental kind of thing. An example is materialism: the only fundamental kind of thing is matter. A materialist might follow Newton and agree that there are also space and forces such as gravity, which are not material things in any plausible sense. Some materialists may thus not be monists. Others may argue that space and forces are not things in their own right, but dependent on matter. Another monism is idealism, argued for by Berkeley, according to whom the concept of matter is inconsistent and to be, for unthinking things, is to be perceived: there are only minds and the ideas in them.
The impression that mental phenomena are very different from physical ones indicates that we tend to endorse dualist intuitions. Not many people nowadays are substance dualists, but it is worth discussing the position in some detail to illustrate problems that any philosophy of mind has to address.
The word ‘substance’ is derived from Latin sub, which means ‘underneath’, and stans, which means ‘standing’. Substances are contrasted with properties or attributes: properties can be predicated of things, but it makes no sense to predicate a substance of something. Substances exemplify or instantiate properties. A substance is that which has properties. The properties of a thing can change, the substance is what remains the same throughout the change: it stands underneath the changing properties and holds them together.
Descartes defines what a substance is in the Replies to the Fourth Set of Objections to his Meditations: ‘The notion of a substance is just this: that it can exist by itself, that it is without the aid of any other substance.’ (Descartes (1984): 159)
2.3 Descartes’s Meditations
In his Meditations, Descartes asks the reader to follow him in his search for safe foundations of knowledge. In the Second Meditation, he establishes the first and most basic certainty: I think, therefore I am. Descartes’s reflections lead him to further insights about what he is. Descartes draws the conclusion that he is a thinking thing. He can exist by himself, as it is possible that everything else does not exist, i.e. his existence does not need the aid of anything else. Given Descartes’s concept of substance, it follows that thinking things are substances. In the Principles of Philosophy, Descartes puts his argument for substance dualism very succinctly: ‘from the mere fact that each of us understands himself to be a thinking thing and is capable, in thought, of excluding from himself every other substance, whether thinking or extended, it is certain that each of us, regarded this way, is really distinct from every other thinking substance and from every corporeal substance.’ (Descartes (1985): Part I, §60, p.213) This establishes the first part of Descartes’s dualism: there are thinking substances.
In the Sixth Meditation, Descartes finally establishes that his perceptions of the physical world are not all illusions created by an evil demon, but largely veridical. It follows that there are things besides thinking things which ‘can exist by themselves’. These are extended things. Thus there are thinking and extended substances. It does not follow quite yet that it is not the same substance that is extended and thinking. Descartes clarifies how he draws this conclusion in the Replies to the Second Set of Objections by adducing a definition: `Two substances are said to be really distinct when each of them can exist apart from the other.’ (Descartes (1984): 114) Extended things can exist without thinking, and, argues Descartes, thinking things can exist without extension, so they can exist apart from each other. It follows that the mind and the body are really distinct substances.
2.4 A Modal Argument
Various arguments for dualism can be extracted from Descartes’s writings. For instance, I can suppose my body to be nothing, but for all that I am still something. It is not a contradiction to suppose that I exist but my body does not. The strongest argument results if we couch it entirely in logical and metaphysical terms.
The Indiscernibility of Identicals is the logical principle that if two things are identical, they have all properties in common. It follows that if two things do not have all properties in common, they are different. The Identity of Indiscernibles, also called Leibniz’s Law, is the converse principle that if two things have all properties in common, they are identical. We won’t appeal to it.
We can show that if two things are identical, then this is necessarily so. Assume that a is identical to b. Evidently, a is identical to itself, and this is necessarily so. Hence a has the property of being necessarily identical to a. But then, by the indiscernibility of identicals, b, too, has the property of being necessarily identical to a, as b is identical to a and identical things have all properties in common. Hence it is necessary that a is identical to b. Furthermore, if something is necessary, then it is true. So ‘a is identical to b’ is equivalent to ‘It is necessary that a is identical to b’.
We can also show that if two things are different, then they are necessarily different, if we make a plausible assumption: if it is possible that a proposition is necessary, then it is necessary. Why is this plausible? A necessary proposition is true in any circumstances whatsoever, or, in Leibniz’s terminology, it is true in all possible worlds. A possible proposition is true in some circumstances, or, in Leibniz’s terminology, it is true in some possible worlds. So if it is possible that a proposition is necessary, then there are possible worlds in which it is necessary. In any such possible world, the proposition is true in all possible worlds, and any such possible world forms part of the totality of possible worlds. So the proposition must be true in all possible worlds, i.e. necessary. Hence if it is possible that a proposition is necessary, it is necessary. Now suppose that a and b are different, i.e. not identical, but assume that it is possible that they are identical. By the first result, a and b are identical if and only if it is necessary that a and b are identical. So if it is possible that a and b are identical, then it is possible that it is necessary that a and b are identical. Then by the plausible assumption, it follows that it is necessary that a and b are identical. What is necessary is true, so a and b are identical. As we assumed that a and b are different, we reached a contradiction. Hence, if a and b are different it is not possible that a and b are identical, i.e. it is necessary that they are different.
We can reformulate Descartes’s argument in these terms. It is possible that I exist without my body, but it is not possible that I exist without my mind, hence by the Indiscernibility of Identicals, my mind and my body are not identical. If my mind and my body are different, then, under a plausible assumption, this is necessarily so. This sounds like as good a basis as anything for substance dualism: if mind and body are necessarily different, then under any circumstances whatsoever could they exist without each other. Necessary difference captures the idea behind Descartes’ definition of substance very well.
2.5 Concluding Remarks
Dualism is often dismissed as implausible. This is a mistake. It is true that very few philosophers nowadays are substance dualists. Nonetheless, as we will see, many accept that there must be mental properties over and above physical properties. Many philosophers of mind are property dualists, even though they are not substance dualists. Many philosophers accept, for instance, that a mental property such as being in pain or knowing where the keys are can be exemplified in various ways by physical properties of creatures, so they cannot be identical to any particular physical properties. It may well be that you and your dog are both in pain or you both know where the keys are, but there are no physical properties you both have in common. The physiology of humans and dogs are very different. Maybe you yourself could have the same mental states exemplified in different physical states. All kinds of things may go on in your body or your brain, but for you the result is just the same feeling of pain. Arguments for property dualism are not so different from Descartes’s, who was of course also a property dualist in addition to being a substance dualist.
If we want to reject substance dualism, we need to work out where Descartes’s argument goes wrong. If Descartes is wrong, then his argument for substance dualism is either invalid or based on a false premise. It is difficult to see what could have gone wrong in the inferences and axioms of the argument. It’s a tough call to contend that the argument is invalid. The fault should lie in the premise it is based on: my mind might have existed without my body. For Descartes’s argument to go through, it suffices that my mind could exist without that particular body or brain it is in fact connected to. This sounds plausible, but maybe we are deluded in thinking that this is a genuine possibility. Maybe we are confused. If the premise is false and our mind is identical to our brain, appearances to the contrary notwithstanding, then we have to bite the bullet that our dualist intuitions are radically misleading about what are genuine possibilities.
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