The term "dualism" can be used for any theory according to which two entities, properties or types of facts are given equal status—that is, neither is taken as completely reducible or explicable in terms of the other. In contemporary philosophy, it refers to a certain view in the philosophy of mind. According to this view, whose clearest proponent is Descartes, mental entities or properties are parts of the universe which are just as fundamental as physical entities or properties. Descartes accepted a form of dualism often called substance dualism, according to which the mind is a special sort of substance, one which could, in principle continue to exist even if all physical substances (e.g. particles) were destroyed.
In the past century, substance dualism has enjoyed relatively little popularity, but property dualism has established a substantial following. According to property dualism, there may be no special mental entities that are capable of existing independently of all physical things, but some entities have certain mental properties (e.g. consciousness) that cannot be reduced to their physical properties (e.g. size and shape).
This article will survey the major dualist positions, and consider some of the main arguments for and against dualism.
Most traditions that accept the existence of a thinking or feeling entity that can survive the death of the physical body can be counted as dualist. Among the ancient Greeks, such positions were common. Plato, in the Phaedo, argued that his rational part would survive the death of his body. His main reasons for the claim seem to stem from his being impressed with the ability of the mind to know eternal, necessary truths. This ability appears to be something beyond the reach of crude matter.
Aristotle's De Anima contains much material that seems to anticipate the contemporary anti-dualist position known as functionalism (see Philosophy of Mind). According to functionalism, mental features simply consist in the physical arrangement of the physical parts of some entity. Aristotle seems to have thought that many aspects of the soul could be understood in terms of the "form" of the organs of the body. Yet, in chapter 4 of book 3, Aristotle notes that the intellect is capable of understanding all things and so must not be 'mixed' with the body. Many of Aristotle's Christian commentators took this suggestion as the acceptance of an immaterial soul.
Both Plato and Aristotle, then, appear to be pulled towards some dualist position on the basis of the type of things that our intellects are capable of grasping. This stands in interesting contrast to the main arguments advanced in favor of dualism in the modern and contemporary periods, discussed below (an interesting exception is the related argument for idealism that Kant provides at the end of his Groundwork for the Metaphysics of Morals).
The great French philosopher René Descartes argued at length for dualism in his Meditations on First Philosophy and Principles of Philosophy. According to Descartes, both our minds and our bodies are equally substances, meaning that they are capable of existing independently of all other entities (with the exception of God), and that "supported" properties. The properties of the body, Descartes thought, were those described in geometrical physics—in particular, extension. The properties of the mind, by contrast, were all at root manifestations of thought. At least in his mature work, however, Descartes recognized that from the claims that the body is an extended thing and that the mind is a thinking thing, it does not yet follow that the mind is a distinct entity from the body. The possibility that remained, in his mind, was that some single thing was both extended and thinking (this is the position now known as property dualism). It was with this possibility in view that he offered the arguments advanced below. Descartes does not appear to have considered the possibility that the property of thinking could itself be a physical property.
Another crucial part of Descartes' substance dualism was his claim that the mind and body were capable of causally affecting each other. It was on this point that much criticism was levied. Given his insistence on the distinct natures of the mind and body, it was hard to see how such distinct things could affect one another (consider, perhaps, how ghosts in most movies are unable to causally interact with the physical world). Indeed, the third proposition proved in Spinoza's Ethics was that things with nothing in common were incapable of causally affecting each other.
Much of continental Europoean metaphysics of the two centuries following Descartes revolved around ways to resolve this exact problem. Occasionalists such as Malebranche claimed that mind and body did not interact, but that God constantly affected each so that they effectly operated as though they interacted. Leibniz and his followers instead claimed that God had 'preprogramed' each substance so as to make them run in parallel, again creating the mere appearance of interaction.
In the nineteenth and early twentieth centuries, dualism was defended by relatively few philosophers. Influential parts of the German philosophical tradition moved towards idealism—the view that the most fundamental substances and properties of the universe were mental, and that physical entities were somehow reducible to ideas or thinking entities. The English-speaking tradition included some idealists (such as J. M. E. McTaggart), but other parts moved towards physicalism—the view that the most fundamental substances and properties of the universe were physical (that is, just those substances and properties describes in physics).
By the mid-twentieth century, dualism was probably at its lowest popularity in history. Yet, in the second half of the century, new advocates of property dualism began to emerge, such as Saul Kripke and David Chalmers. In more recent years, Peter Unger published a book-length defense of substance dualism. The new dualists have devoted much of their philosophical energy to refining earlier arguments for dualism, such as those considered in the next section.
Descartes offered to main arguments for substance dualism, one based on our ability to conceive of physical and mental things existing without one another, and another based on the fact that the body is divisible, whereas the mind seems to be utterly simple. The former has traditionally enjoyed more popularity, and appears to capture a very common intuition; namely, that which we might express with the question "How could mere matter think?" Yet the second argument has had its advocates—indeed, Leibniz took something like it to show that physical entities couldn't be substances in the proper sense, so that the fundamental constituents of reality had to be mental substances.
In rough outline, Descartes' Conceivability Argument runs as follows:
The main point on which this argument has been criticized is that it is hard to find an understanding of 'conceive' which will make premises 1-3 obviously true. Clearly, we seem to have some sort of imaginative ability that extends well beyond what is actually possible. For instance, I can pretend to be a square circle, even though such things are impossible. In that sense, I can certainly imagine the distinctness of my mind and body, but it is clear that substituting "imagine" for "conceive" in premise 3 will yield a false claim.
Descartes himself took the conceivability to be "clear and distinct perception"—a type of conception which was introspectively discernible (that is, one always knows when one is having a clear and distinct perception). He also provided a separate argument in order to show that there was a divine guarantee that whatever he clearly and distinctly perceived was possible. While that way of supporting the opening premises seemed acceptable to many of his contemporaries, later philosophers (even theistically-inclined ones) have found it unconvincing.
This exact issue has received considerable attention in the last half-century, however. One of the major lines of thought (explored in detail by David Chalmers) works by focusing the argument on the case of imagining beings that are physically identical to us, but which lack mental properties (that is, they have no consciousness). Chalmers argued that we can be more confident in the connection between conceivability and possibility in cases like this than in other cases, since what we're conceiving is something very closely tied to conception (that is, consciousness).
Descartes' second argument for dualism is shorter than his first, but for that reason is more difficult to interpret. In outline, it runs as follows:
The chief difficulty with this argument is finding an understanding of "divisible" that will make the premises of the argument acceptable to someone who isn't already convinced of dualism. For instance, if "divisible" means "has parts that can be spatially separated from one another," then premise 2 will be question begging.
At the same time, this argument does seem to be getting at something. My mind appears to be simple in a certain way: it may have different aspects to it, but it does not seem to have (co-temporal) parts that could exist independently of each other. In addition, it seems to be entirely determinate. That is, whereas there may not be exact boundaries in the world between physical things (e.g. there seems to be no exact boundary between my shoulder and my arm), there does seem to be an exact boundary between my mind everything else. This latter line of thought has been recently defended by Peter Unger.
Every worked-out version of substance or property dualist has faced objections that turn on the details of the version. There are, however, certain lines of argument that appear in response to most types of dualism. This section presents two.
Dualism tends to emerge from our sense that the mind is somehow very different than physical things, such as our bodies. Yet, however much inclination we might have to see them as distinct, we have just as much inclination (if not more) to think that they are able to affect one another. Every time you make up your mind and decide to move your body, the background assumption is that your mental decision is capable of participating in a causal chain that ends with the movement of your body. We noted above how this, in conjunction with the thought that things must have something in common in order to interact, made troubles for Descartes. Descartes' dualist successors might have found ways to accept the principle commonality and causation, but their resulting theories strayed very far from common sense (for more, see Rationalism).
It may seem that the fact that the mind and body interact only presents problems for substance dualists, not property dualists. After all, it is open to a property dualist to say that there is just one thing (e.g. your brain) that has both physical and mental properties. There is then no need for a picture where radically distinct entities somehow "touch" each other.
Nevertheless, property dualism faces a more subtle version of the causal objection. Consider an instance where, trying to make sense of fancy-sounding philosophy, you close your eyes to eliminate distraction. Now, if we asked what caused your eyes to close, it looks like there will be two stories. In one story, your eyes closed because certain neural impulses traveled to your eyes, and these impulses came from your brain, which produced these impulses because of some pattern of neural activity. In another story, you decided to close your eyes, and you did so. The objection stems from asking what the relation between these two stories is supposed to be. After all, "deciding to close your eyes" would, for the property dualist, be a mental property of your brain, whereas 'having some pattern of neural activity' would be a physical property. The dualist asserts that these properties are distinct, which seems to mean that these two causal stories have different starting points. But each starting point seems sufficient (intuitively) to cause your eyes to close, yet if both were sufficient, it would seem that only one of them could really be the cause. Yet this seems to be an absurd choice, which some have taken to suggest that the picture provided by property dualism is itself absurd.
The last points of the previous objection point to the central thought behind the current objection. "Naturalism" is the view that the only things that exist are the entities described in the natural sciences (most importantly, physics). This view has some definite attraction: after all, contemporary science has made awe-inspiring strides in understanding many, many aspects of the universe, from atomic physics to the theory of evolution to Neuropsychology. Its predictive power is incredible (as a grim example: who, for most of our history, would have thought that two small spheres of plutonium were capable of mass destruction?).
But this predictive power strongly suggests that natural science is not missing out on any the crucial pieces of reality. For instance, a version of physics which included all the fundamental particles, with the exception of photons, would soon be revealed as predictively inadequate. It would be missing some part of reality, and so would be unable to predict (for instance) the fact that black plastic gets hot on a sunny day. But physics seems, so far at least, to be quite capable of predicting occurrences in the world, and this suggests that it is not missing any of the fundamental constituents (entities or properties) of reality.
Yet, naturalism and both types of dualism are inconsistent. There remain moves for the dualist to make: perhaps the most popular is for the dualist to say that mental properties have some strong relation (such as necessary covariance) with physical properties, but insist that this is less than identity.
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