Here are two essays I have written on the subject, rather than notes. The first focuses more on the metaphysics of dualism and dual-aspect theory, whilst the second focuses on dualism as a theory of mind.
In his quest for knowledge about which we can be absolutely certain, Descartes developed a theory of the nature of our world called Dualism, which proposes that the world consists of two entirely distinct kinds of substance: extended substance (res extensa) and thinking substance (res cogitans). He suggested that our bodies, including our brains, are made up of extended substance, and that our mind is made up of thinking substance. But this left him a problem which he could not satisfactorily resolve - how do the mind and body interact, if they are fundamentally different in nature? His problem can only be resolved when we reject his substantive dualism, and instead work from the premise that mind and body are different, but not different kinds of substance, such that they can still interact.
Descartes developed his substantive dualism through reasoning, and it can be backed up by experiential evidence. He began with the definition of substance offered by Aristotle: that which depends on no other for its existence, meaning that everything else is merely an attribute of some substance. So grass is a substance, whilst green is not, because green can only exist as an attribute of grass, or some other substance. To Descartes, the most important property of a substance, that distinguishes it from all other things, is that it can (in thought experiments, at least) exist independently of anything else.
In his first meditation, Descartes considered the nature of his existence, and he found he could doubt everything about it except that he was thinking. He even doubted he has a body, because he could conceivably be no more than a brain in a scientist's laboratory, or even an electrical charge being carried by some other substance. So he believed that he could, in thought experiments, conceive of himself as nothing but a mind, without a body. This meant that his thoughts could exist independently of his body, and as our body can, for short periods of time whilst unconscious, still function, it is true to suggest that our body can exist independently of our mind (Meditations).
So Descartes concluded that our mind and our body must be two fundamentally different kinds of substance. This idea may be supported by an examination of music. A scientist could take a short piece of music and explain in great detail the physics of the sound waves and how they produce a vibration in our ear which our brain then interprets as sound. But the scientist couldn't explain what the music "sounds" like; he could never experience the music without playing it. Furthermore, that music might be transmitted through the air in sound waves, written down on paper as shapes in ink, or stored on a computer's hard drive as magnetic signatures; here the nature of the extended substances is quite different, and yet the nature of the music remains the same. In other words, the essential qualities of the music have nothing to do with the vibrating molecules that transmit then, or the paper that stores them. So thinking substance and extended substances are quite distinct in their nature.
So there may be two kinds of substance that are distinct, but they cannot separate; rather they are "closely conjoined" since we can think about "pain and other sensations... quite unexpectedly" (Principles). So if they are not separate, and indeed are closely conjoined, they must in some way interact, but how can this be possible if they are fundamentally different? How can something material be affected by something wholly immaterial? Descartes thought that there was a particular section of the brain through which mind and body interact, but that explanation is far from satisfactory as it still doesn't provide an explanation of how the material and immaterial interact.
In fact, when one considers it, it seems wholly inconceivable that such a thing could occur. No satisfactory material analogy might be used here, but it is as absurd as suggesting that a bachelor might be married, or that one equals two. It is simply a logical impossibility, and so it is on this problem that Descartes' dualism fails. But this does not necessarily mean that we should do away with the idea that mind and body might be in some way different, even distinct. I have only shown that we cannot support Descartes' substantive dualism, meaning that we cannot support the notion that mind and body are fundamentally different kinds of substance. But what if they were simply different aspects of the same substance, and so could be distinct and separable, but not substantially different?
This idea was first entertained by Spinoza in his work entitled Ethics. Spinoza was a strong theist, and was interested both in resolving the problems raised by substantive dualism, and in demonstrating the existence of God, and God's "role" in the cosmos. He accepted many of Descartes' arguments, including his idea that while we may use the term "substance" to describe mind and body, they do depend on God to exist. Spinoza disagreed both with the notion of an immaterial mind, and a wholly material cosmos. He believed that not only did both mind and body depend on God's existence, but that they were in fact God's existence, manifested in substance. All of the cosmos is a manifestation of God's essence, and so there is only one substance, God, leaving no room for any substantive dualism.
By this theory, mind and body are one and the same thing - God's essence - but there is room for distinction, as, for Spinoza, they are different attributes of this essence. This dual attribute theory, also called dual aspect theory, seems to resolve Descartes' problem of interaction, because in Spinoza's model there is no interaction. As mind and body are manifestations of the same thing, they need not interact, because they simultaneously share the properties of God's essence.
But this raises its own problems! If everything that exists is the manifestation of this essence, how can one mind by separate from another? As it is quite obvious that we do not all share a very complete collective consciousness, our minds cannot be simultaneously sharing all of God's essence. It seems almost contradictory to suggest that mind and body share this whole essence, and yet different minds have only a subset of this essence. There must either be something external to God in which individuality can reside, or God's essence cannot be shared as a whole.
Similarly, it seems odd to suggest that music might be manifested in its physical and mental attributes, and for the physical attributes to be mutable, and the mental attributes to be immutable. For it follows from Spinoza's conception of God's essence that the essence is mutable, and attributes are immutable manifestations of this essence. So if one attribute seems mutable, then it must be the essence which is mutable, and yet how can the other attribute not be mutable? I cannot see a way to resolve this problem.
In The World as Will and Idea, Schopenhauer developed a slightly more sophisticated dual aspect theory. To begin with, he rejected the notion that God is at the centre of all things, wanting to remove what he saw as speculative theism from philosophy. He thought that the cosmos consisted of manifestations of a universal Will, much like Spinoza's essence, except that it was not God, and its manifestations were subtly different. Schopenhauer borrowed a distinction from Kant to explain the manifestation of the Will: he said that there were two aspects of the Will, nouminal and phenomenal. A nouminal aspect is a manifestation of the Will as it is in itself, independent of a conscious being's knowing it. A phenomenal aspect is the Will as it appears to a conscious being, in relation to our knowledge of its nouminal aspect.
So in the case of music, the nouminal aspect is the essential quality of that music, which is independent of our knowing it, and so to us it is immutable. The phenomenal aspect is then the various ways in which we can know this music, be it in sound waves, stored on paper or magnetic signatures, or any other manifestation. Our minds would then also have both nouminal and phenomenal aspects, and so they are both manifestations of the universal Will, but they do not necessarily simultaneously share the whole of the Will, and so the limits of our knowledge limit the phenomenal aspect, providing scope for separable individuals in the universal Will.
So Schopenhauer allows for a distinction between two kinds of aspect, whilst resolving the problems raised by Spinoza's theory. But his theory does leave the question of how this might fit into our understanding of the physical world, or what Schopenhauer would call the phenomenal world. For we cannot divorce the world from the dimensions of space and time. Substance must exist in these dimensions, as otherwise there can be no explanation for what we experience. Kant firmly believed that only the phenomenal aspects of the Will exist in space and time, and that the nouminal aspects and the Will itself exist outside of these dimensions.
So we might extend Schopenhauer's theory, and conclude that there exists a universal Will, and that in that Will there are discernible individual subsections, each representing something we see in our experiential world. As each of these subsections exist, they are manifested in nouminal aspects, about which we can have no knowledge. The cosmos therefore is a "mass" of nouminal aspects of the Will (I put mass in quotation marks because there is no word that can describe a collection of immaterial things). As conscious beings perceive these nouminal aspects, there pop into existence in our spacial dimensions phenomenal aspects of those subsections of the Will.
We, as individual consciousnesses, experience the Will most directly in ourselves, as our mind is the nouminal aspect of the Will, and our body is the phenomenal aspect of the Will, simultaneously aware of one another and yet with only phenomenal knowledge of oneself. Of others, we are aware only of their phenomenal manifestations, and they are manifested only when a conscious being perceived them. But they continue to exist, outside of spacial dimensions. Whilst this seems like a mind-boggling-ly complex conception of the cosmos, it seems the least objectionable, and coheres with experiential and rational evidence most closely.
Descartes, Meditations on First Philosophy, Cambridge University Press
Descartes, Principles, Cambridge University Press
Schopenhauer, The World as Will and Idea, (in J. Cottingham (ed.), Western Philosophy, Part III, pp. 160-164.)
Spinoza, Ethics, (in J. Cottingham (ed.), Western Philosophy, Part III, pp. 152-154.)
The ontology of the mind has puzzled philosophers and scientists for millennia, providing an unusual diversity of theories. However one theory, or rather one superset of theories, has dominated the discussion since Descartes first proposed it in Meditations. Despite its prominent place in the history of the subject, however, dualism has been the focus of many criticisms, some of which seem to suggest insoluble problems with the distinction between mind and body. These criticisms have, in turn, prompted philosophers to widen the scope of dualism from Descartes' original substantial dualism, creating theories whose objections are as unsubstantiated as the theories themselves.
Descartes' original dualist theory was based upon a few observations he made about the nature of our thoughts. In Meditations he claimed that the only thing about which he can be certain is that he exists, since otherwise he would not have been able to have that thought. From this he determined that he could clearly and distinctly perceive his self (his mind) as distinct from his body, and so suggested that the mind and body are two entirely different and distinct substances, which are closely conjoined so as to allow them to interact. Descartes offered various other observations to support his theory, such as that we may cut off a foot and still have our body, but that we cannot conceivably take away a part of our mind since it is an indivisible whole, and hence the two must be distinctly different kinds of substance. An important part of Descartes' writings on dualism are also based on rationalism, and his emphasis on the creation of knowledge through reason as opposed to pure observation, but such topics are outside the scope of this essay. Suffice to say that Descartes set-up the mind body problem in terms of two distinct substances that interact.
It is in this short definition that we find the most troubling problem with substantial dualism; how can two entirely distinct kinds of substance, material and immaterial, interact? When we talk of interaction between two substances, we generally do so in terms of physics, observable laws of nature, yet many have charged that physics has nothing to say about the immaterial, and that we know of no discoverable causal mechanism between the mind and body.
The first allegation is false, an out of date assumption based upon the Newtonian paradigm, and can be quickly discounted by pointing to work on multiple dimensions, gravity, and other such fields generally associated with quantum relativity. Indeed, the more physicists explore and theorise these contemporary problems, the more they begin to sound like dualists, or even idealists. In one leading theory, loop quantum gravity, things "do not live in space and are not made of matter. Rather their very architecture gives rise to space and matter" (Gefter, Amanda, Throwing Einstein for a Loop, Scientific American, December 2002).
The question of how the material and immaterial might interact is more difficult to solve, however; physics may have a lot to say about the material and immaterial but one cannot generalise based on a mix of conflicting and ever-changing theories. To sustain dualism, its critics charge, one must be able to point to a causal relation between a process or state in the mind and a process or state in the body that ?should in principle be discoverable? (Smith and Jones, 1986, p. 53). That no dualist has been able to do so is made all the more damaging by the fact that a materialist can quite easily provide an alternative explanation; that is that the causal relation between a thought and a resultant action is as simple as changes in one's physical state, in terms of neurological and physical relations and reactions.
But this picture looks less certain that its proponents claim, since the observations we can make may merely be correlations, caused by some other factor. For example, when a barometer needle drops, a thermometer might rise; does this show that one causes the other? No, because in this case, both are caused by an increase in cloud cover. In the same way, the patterns we can observe in the brain may merely correlate with the actions we see in the rest of the body, both being caused by an immaterial mind, or some other agent.
Besides, the fact that we have not yet discovered a causal relation that fits dualism does not mean that one won't be discovered, and hence one cannot say based on the lack of a discovery that such a relation isn't discoverable. Moreover, ?as Kant put it, experience teaches us that a thing is so-and-so, but not that it cannot be otherwise?. Therefore an appeal to monism based upon the lack of experience of the immaterial may be appealing, but it is not proof one way or the other. It does, however, leave us with problems that have no apparent solutions, unless our understanding of physics changes radically.
One can also question the intelligibility of the immaterial affecting the material, as many have done. However this again seems based upon an outdated understanding of physics that has been internalised since school, and is as likely to be due to a lack of understanding as to a lack of any truth in the suggestion. It might have seemed unintelligible to a mediaeval person that the world was a sphere, but we can be fairly certain now that that is true.
Ayer went further to suggest that the very idea of an immaterial entity is unintelligible. First, he said that if we are to call X an entity, we must be able to ask: ?how many Xs have we got??; in this way we can distinguish between, for example, a billiard ball and the property ?red?.One possible reply to this is that it is conceivable that we each have many minds, working together but giving the appearance of a single agent, just as a flock of birds may appear to have a group consciousness. There seems no conceptual reason as to why one cannot apply numerical properties to the immaterial. The many-minds possibility does raise a large number of other problems that monist theories needn't worry about, but again they are not problems that can be solved unless we can solve the problem of the existence and nature of the immaterial.
Ayer's second criticism was that we must be able to individuate between entities, in the way that we can say that two billiard balls are different entities. The only way that we can do so, when inherent physical and subjective properties are discounted, is to refer to their position in space-time, since no two entities can exist in exactly the same place at exactly the same time. If the immaterial does not exist in the dimensions of space-time, then we must ask how we can possibly distinguish between minds? It becomes equally possible that there is only one mind inhabiting many bodies and applying different characteristics to each, but that is not a theory that many dualists would want to sustain, so how can we show that we each have our separate minds, or that, to return to the many-minds theory, that we each have our own single mind? The only solution seems to be to demonstrate a much closer connection between mind and body than Descartes suggested, such that the mind and body share enough properties to be one-body-one-mind but without reducing to a dual-aspect theory.
A solution can be found in the answer to a further problem: at what point do you consider an animal to have a mind, rather than pure behavioural instincts? The materialists reply would be that it will depend upon the capacity of the central nervous system to hold experiences sufficiently complex to allow for consciousness, and psychological mechanisms to translate these into thoughts (in terms of language). In other words, particularly advanced configurations of matter can provide the means for consciousness. An analogous answer can be given by the dualist; that particularly advanced configurations of mind can provide the means for consciousness. But if the dualist is correct here, what of the correlation in the complexity of the brain? To reduce the brain to a mere physical container seems strange.
An answer can be found in property or panpsychic dualism, which hold that all matter has immaterial, mental properties, closely tied to the physical properties, so that particular configurations of mental or physical entities will necessarily entail a correlative configuration in the opposing property. This not only provides as unobjectionable an answer to the question of consciousness as the materialist's answer, but it also provides a close connection between mind and body, such that each individual will only have one mind, and one that can be individuated from other minds, by virtue of the fact that the mind is the configuration of the mental properties of the body. According to its original proponent, this position is ?an innocent version of dualism, entirely compatible with the scientific view of the world?since ?nothing in this approach contradicts anything in physical theory; we simply need to add further bridging principles to explain how experience arises from physical processes?.
No matter how innocent the position is, however, it does not solve, many of the problems associated with dualism. It cannot say with certainty whether or not matter has mental properties, nor how these mental properties might interact with the physical properties; as with all dualist theories, it can only offer conceivable possibilities, and contrast those with opposing monist theories with a view to making dualism seem more attractive. On the other hand, materialism is afflicted by many of the same problems, and many more of its own, and so it is not yet proven either. In other words, neither dualism nor monism are proven, and both are faced with tough problems, some of which may be resolved or tempered by the discoveries of scientists, and others which we are unlikely to ever answer, unless we discover a way to measure that which is currently unobservable, if indeed it exists: the immaterial.
But to say that the problems are tough is not to say that they are insoluble, rather that we are unlikely to solve them any time soon. In claiming that the problems of dualism are insoluble, one would be implying that it is a position that cannot be sustained, and that is a conclusion that cannot itself be sustained.
Chalmers, David, Facing Up to the Problem of Consciousness, Journal of Consciousness Studies, 1995
Descartes, Meditations on First Philosophy, ed. John Cottingham, Cambridge University Press, 1996
Hart, W. D., Engines of the Soul, Cambridge University Press, 1988
Heil, John, Philosophy of Mind: A Contemporary Introduction, London: Routledge, 1998
Smith, P &Jones, O. R, The Philosophy of Mind: An Introduction, Cambridge University Press, 1986