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Arguments for Dualism

The Knowledge Argument Against Physicalism

One category of arguments for dualism is constituted by the standard objections against physicalism. Prime examples are those based on the existence of qualia, the most important of which is the so-called ‘knowledge argument’. Because this argument has its own entry (see the entry qualia: the knowledge argument), I shall deal relatively briefly with it here. One should bear in mind, however, that all arguments against physicalism are also arguments for the irreducible and hence immaterial nature of the mind and, given the existence of the material world, are thus arguments for dualism.

The knowledge argument asks us to imagine a future scientist who has lacked a certain sensory modality from birth, but who has acquired a perfect scientific understanding of how this modality operates in others. This scientist—call him Harpo—may have been born stone deaf, but become the world’s greatest expert on the machinery of hearing: he knows everything that there is to know within the range of the physical and behavioural sciences about hearing. Suppose that Harpo, thanks to developments in neurosurgery, has an operation which finally enables him to hear. It is suggested that he will then learn something he did not know before, which can be expressed as what it is like to hear, or the qualitative or phenomenal nature of sound. These qualitative features of experience are generally referred to as qualia. If Harpo learns something new, he did not know everything before. He knew all the physical facts before. So what he learns on coming to hear—the facts about the nature of experience or the nature of qualia—are non-physical. This establishes at least a state or property dualism. (See Jackson 1982; Robinson 1982.)

There are at least two lines of response to this popular but controversial argument. First is the ‘ability’ response. According to this, Harpo does not acquire any new factual knowledge, only ‘knowledge how’, in the form of the ability to respond directly to sounds, which he could not do before. This essentially behaviouristic account is exactly what the intuition behind the argument is meant to overthrow. Putting ourselves in Harpo’s position, it is meant to be obvious that what he acquires is knowledge of what something is like, not just how to do something. Such appeals to intuition are always, of course, open to denial by those who claim not to share the intuition. Some ability theorists seem to blur the distinction between knowing what something is like and knowing how to do something, by saying that the ability Harpo acquires is to imagine or remember the nature of sound. In this case, what he acquires the ability to do involves the representation to himself of what the thing is like. But this conception of representing to oneself, especially in the form of imagination, seems sufficiently close to producing in oneself something very like a sensory experience that it only defers the problem: until one has a physicalist gloss on what constitutes such representations as those involved in conscious memory and imagination, no progress has been made.

The other line of response is to argue that, although Harpo’s new knowledge is factual, it is not knowledge of a new fact. Rather, it is new way of grasping something that he already knew. He does not realise this, because the concepts employed to capture experience (such as ‘looks red’ or ‘sounds C-sharp’) are similar to demonstratives, and demonstrative concepts lack the kind of descriptive content that allow one to infer what they express from other pieces of information that one may already possess. A total scientific knowledge of the world would not enable you to say which time was ‘now’ or which place was ‘here’. Demonstrative concepts pick something out without saying anything extra about it. Similarly, the scientific knowledge that Harpo originally possessed did not enable him to anticipate what it would be like to re-express some parts of that knowledge using the demonstrative concepts that only experience can give one. The knowledge, therefore, appears to be genuinely new, whereas only the mode of conceiving it is novel.

Proponents of the epistemic argument respond that it is problematic to maintain both that the qualitative nature of experience can be genuinely novel, and that the quality itself be the same as some property already grasped scientifically: does not the experience’s phenomenal nature, which the demonstrative concepts capture, constitute a property in its own right? Another way to put this is to say that phenomenal concepts are not pure demonstratives, like ‘here’ and ‘now’, or ‘this’ and ‘that’, because they do capture a genuine qualitative content. Furthermore, experiencing does not seem to consist simply in exercising a particular kind of concept, demonstrative or not. When Harpo has his new form of experience, he does not simply exercise a new concept; he also grasps something new—the phenomenal quality—with that concept. How decisive these considerations are, remains controversial.

The Argument from Predicate Dualism to Property Dualism

I said above that predicate dualism might seem to have no ontological consequences, because it is concerned only with the different way things can be described within the contexts of the different sciences, not with any real difference in the things themselves. This, however, can be disputed.

The argument from predicate to property dualism moves in two steps, both controversial. The first claims that the irreducible special sciences, which are the sources of irreducible predicates, are not wholly objective in the way that physics is, but depend for their subject matter upon interest-relative perspectives on the world. This means that they, and the predicates special to them, depend on the existence of minds and mental states, for only minds have interest-relative perspectives. The second claim is that psychology—the science of the mental—is itself an irreducible special science, and so it, too, presupposes the existence of the mental. Mental predicates therefore presuppose the mentality that creates them: mentality cannot consist simply in the applicability of the predicates themselves.

First, let us consider the claim that the special sciences are not fully objective, but are interest-relative.

No-one would deny, of course, that the very same subject matter or ‘hunk of reality’ can be described in irreducibly different ways and it still be just that subject matter or piece of reality. A mass of matter could be characterized as a hurricane, or as a collection of chemical elements, or as mass of sub-atomic particles, and there be only the one mass of matter. But such different explanatory frameworks seem to presuppose different perspectives on that subject matter.

This is where basic physics, and perhaps those sciences reducible to basic physics, differ from irreducible special sciences. On a realist construal, the completed physics cuts physical reality up at its ultimate joints: any special science which is nomically strictly reducible to physics also, in virtue of this reduction, it could be argued, cuts reality at its joints, but not at its minutest ones. If scientific realism is true, a completed physics will tell one how the world is, independently of any special interest or concern: it is just how the world is. It would seem that, by contrast, a science which is not nomically reducible to physics does not take its legitimation from the underlying reality in this direct way. Rather, such a science is formed from the collaboration between, on the one hand, objective similarities in the world and, on the other, perspectives and interests of those who devise the science. The concept of hurricane is brought to bear from the perspective of creatures concerned about the weather. Creatures totally indifferent to the weather would have no reason to take the real patterns of phenomena that hurricanes share as constituting a single kind of thing. With the irreducible special sciences, there is an issue of salience , which involves a subjective component: a selection of phenomena with a certain teleology in mind is required before their structures or patterns are reified. The entities of metereology or biology are, in this respect, rather like Gestalt phenomena.

Even accepting this, why might it be thought that the perspectivality of the special sciences leads to a genuine property dualism in the philosophy of mind? It might seem to do so for the following reason. Having a perspective on the world, perceptual or intellectual, is a psychological state. So the irreducible special sciences presuppose the existence of mind. If one is to avoid an ontological dualism, the mind that has this perspective must be part of the physical reality on which it has its perspective. But psychology, it seems to be almost universally agreed, is one of those special sciences that is not reducible to physics, so if its subject matter is to be physical, it itself presupposes a perspective and, hence, the existence of a mind to see matter as psychological. If this mind is physical and irreducible, it presupposes mind to see it as such. We seem to be in a vicious circle or regress.

We can now understand the motivation for full-blown reduction. A true basic physics represents the world as it is in itself, and if the special sciences were reducible, then the existence of their ontologies would make sense as expressions of the physical, not just as ways of seeing or interpreting it. They could be understood ‘from the bottom up’, not from top down. The irreducibility of the special sciences creates no problem for the dualist, who sees the explanatory endeavor of the physical sciences as something carried on from a perspective conceptually outside of the physical world. Nor need this worry a physicalist, if he can reduce psychology, for then he could understand ‘from the bottom up’ the acts (with their internal, intentional contents) which created the irreducible ontologies of the other sciences. But psychology is one of the least likely of sciences to be reduced. If psychology cannot be reduced, this line of reasoning leads to real emergence for mental acts and hence to a real dualism for the properties those acts instantiate (Robinson 2003).

The Modal Argument

There is an argument, which has roots in Descartes (Meditation VI), which is a modal argument for dualism. One might put it as follows:

It is imaginable that one’s mind might exist without one’s body.
therefore

It is conceivable that one’s mind might exist without one’s body.
therefore

It is possible one’s mind might exist without one’s body.
therefore

One’s mind is a different entity from one’s body.
The rationale of the argument is a move from imaginability to real possibility. I include (2) because the notion of conceivability has one foot in the psychological camp, like imaginability, and one in the camp of pure logical possibility and therefore helps in the transition from one to the other.

This argument should be distinguished from a similar ‘conceivability’ argument, often known as the ‘zombie hypothesis’, which claims the imaginability and possibility of my body (or, in some forms, a body physically just like it) existing without there being any conscious states associated with it. (See, for example, Chalmers (1996), 94–9.) This latter argument, if sound, would show that conscious states were something over and above physical states. It is a different argument because the hypothesis that the unaltered body could exist without the mind is not the same as the suggestion that the mind might continue to exist without the body, nor are they trivially equivalent. The zombie argument establishes only property dualism and a property dualist might think disembodied existence inconceivable—for example, if he thought the identity of a mind through time depended on its relation to a body (e.g., Penelhum 1970).

Before Kripke (1972/80), the first challenge to such an argument would have concerned the move from (3) to (4). When philosophers generally believed in contingent identity, that move seemed to them invalid. But nowadays that inference is generally accepted and the issue concerns the relation between imaginability and possibility. No-one would nowadays identify the two (except, perhaps, for certain quasi-realists and anti-realists), but the view that imaginability is a solid test for possibility has been strongly defended. W. D. Hart ((1994), 266), for example, argues that no clear example has been produced such that “one can imagine that p (and tell less imaginative folk a story that enables them to imagine that p) plus a good argument that it is impossible that p. No such counterexamples have been forthcoming…” This claim is at least contentious. There seem to be good arguments that time-travel is incoherent, but every episode of Star-Trek or Doctor Who shows how one can imagine what it might be like were it possible.

It is worth relating the appeal to possibility in this argument to that involved in the more modest, anti-physicalist, zombie argument. The possibility of this hypothesis is also challenged, but all that is necessary for a zombie to be possible is that all and only the things that the physical sciences say about the body be true of such a creature. As the concepts involved in such sciences—e.g., neuron, cell, muscle—seem to make no reference, explicit or implicit, to their association with consciousness, and are defined in purely physical terms in the relevant science texts, there is a very powerful prima facie case for thinking that something could meet the condition of being just like them and lack any connection with consciousness. There is no parallel clear, uncontroversial and regimented account of mental concepts as a whole that fails to invoke, explicitly or implicitly, physical (e.g., behavioural) states.

For an analytical behaviourist the appeal to imaginability made in the argument fails, not because imagination is not a reliable guide to possibility, but because we cannot imagine such a thing, as it is a priori impossible. The impossibility of disembodiment is rather like that of time travel, because it is demonstrable a priori, though only by arguments that are controversial. The argument can only get under way for those philosophers who accept that the issue cannot be settled a priori, so the possibility of the disembodiment that we can imagine is still prima facie open.

A major rationale of those who think that imagination is not a safe indication of possibility, even when such possibility is not eliminable a priori, is that we can imagine that a posteriori necessities might be false—for example, that Hesperus might not be identical to Phosphorus. But if Kripke is correct, that is not a real possibility. Another way of putting this point is that there are many epistemic possibilities which are imaginable because they are epistemic possibilities, but which are not real possibilities. Richard Swinburne (1997, New Appendix C), whilst accepting this argument in general, has interesting reasons for thinking that it cannot apply in the mind-body case. He argues that in cases that involve a posteriori necessities, such as those identities that need discovering, it is because we identify those entities only by their ‘stereotypes’ (that is, by their superficial features observable by the layman) that we can be wrong about their essences. In the case of our experience of ourselves this is not true.

Now it is true that the essence of Hesperus cannot be discovered by a mere thought experiment. That is because what makes Hesperus Hesperus is not the stereotype, but what underlies it. But it does not follow that no one can ever have access to the essence of a substance, but must always rely for identification on a fallible stereotype. One might think that for the person him or herself, while what makes that person that person underlies what is observable to others, it does not underlie what is experienceable by that person, but is given directly in their own self-awareness.

This is a very appealing Cartesian intuition: my identity as the thinking thing that I am is revealed to me in consciousness, it is not something beyond the veil of consciousness. Now it could be replied to this that though I do access myself as a conscious subject, so classifying myself is rather like considering myself qua cyclist. Just as I might never have been a cyclist, I might never have been conscious, if things had gone wrong in my very early life. I am the organism, the animal, which might not have developed to the point of consciousness, and that essence as animal is not revealed to me just by introspection.

But there are vital differences between these cases. A cyclist is explicitly presented as a human being (or creature of some other animal species) cycling: there is no temptation to think of a cyclist as a basic kind of thing in its own right. Consciousness is not presented as a property of something, but as the subject itself. Swinburne’s claim that when we refer to ourselves we are referring to something we think we are directly aware of and not to ‘something we know not what’ that underlies our experience seemingly ‘of ourselves’ has powerful intuitive appeal and could only be overthrown by very forceful arguments. Yet, even if we are not referring primarily to a substrate, but to what is revealed in consciousness, could it not still be the case that there is a necessity stronger than causal connecting this consciousness to something physical? To consider this further we must investigate what the limits are of the possible analogy between cases of the water-H2O kind, and the mind-body relation.

We start from the analogy between the water stereotype—how water presents itself—and how consciousness is given first-personally to the subject. It is plausible to claim that something like water could exist without being H2O, but hardly that it could exist without some underlying nature. There is, however, no reason to deny that this underlying nature could be homogenous with its manifest nature: that is, it would seem to be possible that there is a world in which the water-like stuff is an element, as the ancients thought, and is water-like all the way down. The claim of the proponents of the dualist argument is that this latter kind of situation can be known to be true a priori in the case of the mind: that is, one can tell by introspection that it is not more-than-causally dependent on something of a radically different nature, such as a brain or body. What grounds might one have for thinking that one could tell that a priori?

The only general argument that seem to be available for this would be the principle that, for any two levels of discourse, A and B, they are more-than-causally connected only if one entails the other a priori. And the argument for accepting this principle would be that the relatively uncontroversial cases of a posteriori necessary connections are in fact cases in which one can argue a priori from facts about the microstructure to the manifest facts. In the case of water, for example, it would be claimed that it follows a priori that if there were something with the properties attributed to H2O by chemistry on a micro level, then that thing would possess waterish properties on a macro level. What is established a posteriori is that it is in fact H2O that underlies and explains the waterish properties round here, not something else: the sufficiency of the base—were it to obtain—to explain the phenomena, can be deduced a priori from the supposed nature of the base. This is, in effect, the argument that Chalmers uses to defend the zombie hypothesis. The suggestion is that the whole category of a posteriori more-than-causally necessary connections (often identified as a separate category of metaphysical necessity) comes to no more than this. If we accept that this is the correct account of a posteriori necessities, and also deny the analytically reductionist theories that would be necessary for a priori connections between mind and body, as conceived, for example, by the behaviourist or the functionalist, does it follow that we can tell a priori that consciousness is not more-than-causally dependent on the body?

It is helpful in considering this question to employ a distinction like Berkeley’s between ideas and notions. Ideas are the objects of our mental acts, and they capture transparently—‘by way of image or likeness’ (Principles, sect. 27)—that of which they are the ideas. The self and its faculties are not the objects of our mental acts, but are captured only obliquely in the performance of its acts, and of these Berkeley says we have notions, meaning by this that what we capture of the nature of the dynamic agent does not seem to have the same transparency as what we capture as the normal objects of the agent’s mental acts. It is not necessary to become involved in Berkeley’s metaphysics in general to feel the force of the claim that the contents and internal objects of our mental acts are grasped with a lucidity that exceeds that of our grasp of the agent and the acts per se. Because of this, notions of the self perhaps have a ‘thickness’ and are permanently contestable: there seems always to be room for more dispute as to what is involved in that concept. (Though we shall see later, in 5.2.2, that there is a ‘non-thick’ way of taking the Berkeleyan concept of a notion.)

Because ‘thickness’ always leaves room for dispute, this is one of those cases in philosophy in which one is at the mercy of the arguments philosophers happen to think up. The conceivability argument creates a prima facie case for thinking that mind has no more than causal ontological dependence on the body. Let us assume that one rejects analytical (behaviourist or functionalist) accounts of mental predicates. Then the above arguments show that any necessary dependence of mind on body does not follow the model that applies in other scientific cases. This does not show that there may not be other reasons for believing in such dependence, for so many of the concepts in the area are still contested. For example, it might be argued that identity through time requires the kind of spatial existence that only body can give: or that the causal continuity required by a stream of consciousness cannot be a property of mere phenomena. All these might be put forward as ways of filling out those aspects of our understanding of the self that are only obliquely, not transparently, presented in self-awareness. The dualist must respond to any claim as it arises: the conceivability argument does not pre-empt them.

Arguments from Personal Identity

There is a long tradition, dating at least from Reid (1785/1969), for arguing that the identity of persons over time is not a matter of convention or degree in the way that the identity of other (complex) substances is and that this shows that the self is a different kind of entity from any physical body. Criticism of these arguments and of the intuitions on which they rest, running from Hume to Parfit (1970: 1984), have left us with an inconclusive clash of intuitions. The argument under consideration and which, possibly, has its first statement in Madell (1981), does not concern identity through time, but the consequences for identity of certain counterfactuals concerning origin. It can, perhaps, therefore, break the stalemate which faces the debate over diachronic identity. The claim is that the broadly conventionalist ways which are used to deal with problem cases through time for both persons and material objects, and which can also be employed in cases of counterfactuals concerning origin for bodies, cannot be used for similar counterfactuals concerning persons or minds.

Concerning ordinary physical objects, it is easy to imagine counterfactual cases where questions of identity become problematic. Take the example of a particular table. We can scale counterfactual suggestions as follows:

This table might have been made of ice.
This table might have been made of a different sort of wood.
This table might have been made of 95% of the wood it was made of and 5% of some other wood.
The first suggestion would normally be rejected as clearly false, but there will come a point along the spectrum illustrated by (i) and (iii) and towards (iii) where the question of whether the hypothesised table would be the same as the one that actually exists have no obvious answer. It seems that the question of whether it ‘really’ is the same one has no clear meaning: it is of, say, 75% the same matter and of 25% different matter; these are the only genuine facts in the case; the question of numerical identity can be decided in any convenient fashion, or left unresolved. There will thus be a penumbra of counterfactual cases where the question of whether two things would be the same is not a matter of fact.

Let us now apply this thought to conscious subjects. Suppose that a given human individual had had origins different from those which he in fact had such that whether that difference affected who he was was not obvious to intuition. What would count as such a case might be a matter of controversy, but there must be one. Perhaps it is unclear whether, if there had been a counterpart to Jones’ body from the same egg but a different though genetically identical sperm from the same father, the person there embodied would have been Jones. Some philosophers might regard it as obvious that sameness of sperm is essential to the identity of a human body and to personal identity. In that case imagine a counterpart sperm in which some of the molecules in the sperm are different; would that be the same sperm? If one pursues the matter far enough there will be indeterminacy which will infect that of the resulting body. There must therefore be some difference such that neither natural language nor intuition tells us whether the difference alters the identity of the human body; a point, that is, where the question of whether we have the same body is not a matter of fact.

How one is to describe these cases is, in some respects, a matter of controversy. Some philosophers think one can talk of vague identity or partial identity. Others think that such expressions are nonsensical. There is no space to discuss this issue here. It is enough to assume, however, that questions of how one is allowed to use the concept of identity effect only the care with which one should characterize these cases, not any substantive matter of fact. There are cases of substantial overlap of constitution in which that fact is the only bedrock fact in the case: there is no further fact about whether they are ‘really’ the same object. If there were, then there would have to be a haecceitas or thisness belonging to and individuating each complex physical object, and this I am assuming to be implausible if not unintelligible. (More about the conditions under which haecceitas can make sense will be found below.)

One might plausibly claim that no similar overlap of constitution can be applied to the counterfactual identity of minds. In Geoffrey Madell’s (1981) words:

But while my present body can thus have its partial counterpart in some possible world, my present consciousness cannot. Any present state of consciousness that I can imagine either is or is not mine. There is no question of degree here. (91)
Why is this so? Imagine the case where we are not sure whether it would have been Jones’ body—and, hence, Jones—that would have been created by the slightly modified sperm and the same egg. Can we say, as we would for an object with no consciousness, that the story something the same, something different is the whole story: that overlap of constitution is all there is to it? For the Jones body as such, this approach would do as well as for any other physical object. But suppose Jones, in reflective mood, asks himself ‘if that had happened, would I have existed?’ There are at least three answers he might give to himself. (i) I either would or would not, but I cannot tell. (ii) There is no fact of the matter whether I would or would not have existed: it is just a mis-posed question. (iii) In some ways, or to some degree, I would have, and in some ways, or to some degree, I would not. The creature who would have existed would have had a kind of overlap of psychic constitution with me.

The third answer parallels the response we would give in the case of bodies. But as an account of the subjective situation, it is arguable that this makes no sense. Call the creature that would have emerged from the slightly modified sperm, ‘Jones2’. Is the overlap suggestion that, just as, say 85% of Jones2’s original body would have been identical with Jones’, about 85% of his psychic life would have been Jones’? That it would have been like Jones’—indeed that Jones2 might have had a psychic life 100% like Jones’—makes perfect sense, but that he might have been to that degree, the same psyche—that Jones ‘85% existed’ —arguably makes no sense. Take the case in which Jones and Jones2 have exactly similar lives throughout: which 85% of the 100% similar mental events do they share? Nor does it make sense to suggest that Jones might have participated in the whole of Jones2’s psychic life, but in a rather ghostly only 85% there manner. Clearly, the notion of overlap of numerically identical psychic parts cannot be applied in the way that overlap of actual bodily part constitution quite unproblematically can.

This might make one try the second answer. We can apply the ‘overlap’ answer to the Jones body, but the question of whether the minds or subjects would have been the same, has no clear sense. It is difficult to see why it does not. Suppose Jones found out that he had originally been one of twins, in the sense that the zygote from which he developed had divided, but that the other half had died soon afterwards. He can entertain the thought that if it had been his half that had died, he would never have existed as a conscious being, though someone would whose life, both inner and outer, might have been very similar to his. He might feel rather guiltily grateful that it was the other half that died. It would be strange to think that Jones is wrong to think that there is a matter of fact about this. And how is one to ‘manage’ the transition from the case where there is a matter of fact to the case where there is not?

If the reasoning above is correct, one is left with only the first option. If so, there has to be an absolute matter of fact from the subjective point of view. But the physical examples we have considered show that when something is essentially complex, this cannot be the case. When there is constitution, degree and overlap of constitution are inevitably possible. So the mind must be simple, and this is possible only if it is something like a Cartesian substance.

The Aristotelian Argument in a Modern Form

Putting his anti-materialist argument outlined above, in section 1, in very general terms, Aristotle’s worry was that a material organ could not have the range and flexibility that are required for human thought. His worries concerned the cramping effect that matter would have on the range of objects that intellect could accommodate. Parallel modern concerns centre on the restriction that matter would impose on the range of rational processes that we could exhibit. Godel, for example, believed that his famous theorem showed that there are demonstrably rational forms of mathematical thought of which humans are capable which could not be exhibited by a mechanical or formal system of a sort that a physical mind would have to be. Penrose (1990) has argued that Turing’s halting problem has similar consequences. In general, the fear is that the materialist monist has to treat the organ of thought as, what Dennett (1987:61) calls, a syntactic engine: that is, as something that operates without any fundamental reference to the propositional content of what it thinks. It works as a machine that only shadows the pattern of meaning. But it is hard to convince oneself that, as one, for example, reflectively discusses philosophy and struggles to follow what is being said, that it is not the semantic content that is driving one’s responses. But if we are truly semantic engines, it is difficult to see how we can avoid at least a property dualism. These issues are, of course, connected with problems raised by Brentano, concerning the irreducibility of intentionality. Despite the interest of the arguments for dualism based on the irreducible flexibility of intellect, most of the modern debate turns on arguments that have a Cartesian origin.

Problems for Dualism

We have already discussed the problem of interaction. In this section we shall consider two other facets of dualism that worry critics. First, there is what one might term the queerness of the mental if conceived of as non-physical. Second there is the difficulty of giving an account of the unity of the mind. We shall consider this latter as it faces both the bundle theorist and the substance dualist.

The Queerness of the Mental

Mental states are characterised by two main properties, subjectivity, otherwise known as privileged access, and intentionality. Physical objects and their properties are sometimes observable and sometimes not, but any physical object is equally accessible, in principle, to anyone. From the right location, we could all see the tree in the quad, and, though none of us can observe an electron directly, everyone is equally capable of detecting it in the same ways using instruments. But the possessor of mental states has a privileged access to them that no-one else can share. That is why there is a sceptical ‘problem of other minds’, but no corresponding ‘problem of my own mind’. This suggests to some philosophers that minds are not ordinary occupants of physical space.

Physical objects are spatio-temporal, and bear spatio-temporal and causal relations to each other. Mental states seem to have causal powers, but they also possess the mysterious property of intentionality—being about other things—including things like Zeus and the square root of minus one, which do not exist. No mere physical thing could be said to be, in a literal sense, ‘about’ something else. The nature of the mental is both queer and elusive. In Ryle’s deliberately abusive phrase, the mind, as the dualist conceives of it, is a ‘ghost in a machine’. Ghosts are mysterious and unintelligible: machines are composed of identifiable parts and work on intelligible principles. But this contrast holds only if we stick to a Newtonian and common-sense view of the material. Think instead of energy and force-fields in a space-time that possesses none of the properties that our senses seem to reveal: on this conception, we seem to be able to attribute to matter nothing beyond an abstruse mathematical structure. Whilst the material world, because of its mathematicalisation, forms a tighter abstract system than mind, the sensible properties that figure as the objects of mental states constitute the only intelligible content for any concrete picture of the world that we can devise. Perhaps the world within the experiencing mind is, once one considers it properly, no more—or even less—queer than the world outside it.

The Unity of the Mind

Whether one believes that the mind is a substance or just a bundle of properties, the same challenge arises, which is to explain the nature of the unity of the immaterial mind. For the Cartesian, that means explaining how he understands the notion of immaterial substance. For the Humean, the issue is to explain the nature of the relationship between the different elements in the bundle that binds them into one thing. Neither tradition has been notably successful in this latter task: indeed, Hume, in the appendix to the Treatise, declared himself wholly mystified by the problem, rejecting his own initial solution (though quite why is not clear from the text).

Unity and Bundle Dualism

If the mind is only a bundle of properties, without a mental substance to unite them, then an account is needed of what constitutes its unity. The only route appears to be to postulate a primitive relation of co-consciousness in which the various elements stand to each other.

There are two strategies which can be used to attack the bundle theory. One is to claim that our intuitions favour belief in a subject and that the arguments presented in favour of the bundle alternative are unsuccessful, so the intuition stands. The other is to try to refute the theory itself. Foster (1991, 212–9) takes the former path. This is not effective against someone who thinks that metaphysical economy gives a prima facie priority to bundle theories, on account of their avoiding mysterious substances.

The core objection to bundle theories (see, for example, Armstrong (1968), 21–3) is that, because it takes individual mental contents as its elements, such contents should be able to exist alone, as could the individual bricks from a house. Hume accepted this consequence, but most philosophers regard it as absurd. There could not be a mind that consisted of a lone pain or red after-image, especially not of one that had detached itself from the mind to which it had previously belonged. Therefore it makes more sense to think of mental contents as modes of a subject.

Bundle theorists tend to take phenomenal contents as the primary elements in their bundle. Thus the problem is how to relate, say, the visual field to the auditory field, producing a ‘unity of apperception’, that is, a total experience that seems to be presented to a single subject. Seeing the problem in this way has obvious Humean roots. This atomistic conception of the problem becomes less natural if one tries to accommodate other kinds of mental activity and contents. How are acts of conceptualising, attending to or willing with respect to, such perceptual contents to be conceived? These kinds of mental acts seem to be less naturally treated as atomic elements in a bundle, bound by a passive unity of apperception. William James (1890, vol. 1, 336–41) attempts to answer these problems. He claims to introspect in himself a ‘pulse of thought’ for each present moment, which he calls ‘the Thought’ and which is the ‘vehicle of the judgement of identity’ and the ‘vehicle of choice as well as of cognition’. These ‘pulses’ are united over time because each ‘appropriates’ the past Thoughts and ‘makes us say “as sure as I exist, those past facts were part of myself”. James attributes to these Thoughts acts of judging, attending, willing etc, and this may seem incoherent in the absence of a genuine subject. But there is also a tendency to treat many if not all aspects of agency as mere awareness of bodily actions or tendencies, which moves one back towards a more normal Humean position. Whether James’ position really improves on Hume’s, or merely mystifies it, is still a moot point. (But see Sprigge (1993), 84–97, for an excellent, sympathetic discussion.)

Unity and Substance Dualism

The problem is to explain what kind of a thing an immaterial substance is, such that its presence explains the unity of the mind. The answers given can be divided into three kinds.

(a) The ‘ectoplasm’ account: The view that immaterial substance is a kind of immaterial stuff. There are two problems with this approach. First, in so far as this ‘ectoplasm’ has any characterisation as a ‘stuff’—that is, a structure of its own over and above the explicitly mental properties that it sustains—it leaves it as much a mystery why this kind of stuff should support consciousness as it is why ordinary matter should. Second, and connectedly, it is not clear in what sense such stuff is immaterial, except in the sense that it cannot be integrated into the normal scientific account of the physical world. Why is it not just an aberrant kind of physical stuff?

(b) The ‘consciousness’ account: The view that consciousness is the substance. Account (a) allowed the immaterial substance to have a nature over and above the kinds of state we would regard as mental. The consciousness account does not. This is Descartes’ view. The most obvious objection to this theory is that it does not allow the subject to exist when unconscious. This forces one to take one of four possible theories. One could claim (i) that we are conscious when we do not seem to be (which was Descartes’ view): or (ii) that we exist intermittently, though are still the same thing (which is Swinburne’s theory, (1997), 179): or (iii) that each of us consists of a series of substances, changed at any break in consciousness, which pushes one towards a constructivist account of identity through time and so towards the spirit of the bundle theory: or (iv) even more speculatively, that the self stands in such a relation to the normal time series that its own continued existence is not brought into question by its failure to be present in time at those moments when it is not conscious within that series (Robinson, forthcoming).

(c) The ‘no-analysis’ account: The view that it is a mistake to present any analysis. This is Foster’s view, though I think Vendler (1984) and Madell (1981) have similar positions. Foster argues that even the ‘consciousness’ account is an attempt to explain what the immaterial self is ‘made of’ which assimilates it too far towards a kind of physical substance. In other words, Descartes has only half escaped from the ‘ectoplasmic’ model. (He has half escaped because he does not attribute non-mental properties to the self, but he is still captured by trying to explain what it is made of.)

Foster (1991) expresses it as follows:

…it seems to me that when I focus on myself introspectively, I am not only aware of being in a certain mental condition; I am also aware, with the same kind of immediacy, of being a certain sort of thing…
It will now be asked: ‘Well, what is this nature, this sortal attribute? Let’s have it specified!’ But such a demand is misconceived. Of course, I can give it a verbal label: for instance, I can call it ‘subjectness’ or ‘selfhood’. But unless they are interpreted ‘ostensively’, by reference to what is revealed by introspective awareness, such labels will not convey anything over and above the nominal essence of the term ‘basic subject’. In this respect, however, there is no difference between this attribute, which constitutes the subject’s essential nature, and the specific psychological attributes of his conscious life…

Admittedly, the feeling that there must be more to be said from a God’s eye view dies hard. The reason is that, even when we have acknowledged that basic subjects are wholly non-physical, we still tend to approach the issue of their essential natures in the shadow of the physical paradigm. (243–5)

Berkeley’s concept of notion again helps here. One can interpret Berkeley as implying that there is more to the self than introspection can capture, or we can interpret him as saying that notions, though presenting stranger entities than ideas, capture them just as totally. The latter is the ‘no account is needed’ view.

The Case Against Physicalism I: Qualia and Consciousness
Having provided an answer to the interpretation question, I now turn to the truth question: is physicalism (as we have interpreted it so far) true? I will first discuss three reasons for supposing that physicalism is not true. Then I will consider the case for physicalism.

The main argument against physicalism is usually thought to concern the notion of qualia, the felt qualities of experience. The notion of qualia raises puzzles of its own, puzzles having to do with its connection to other notions such as consciousness, introspection, epistemic access, acquaintance, the first-person perspective and so on. However the idea that we will discuss here is the apparent contradiction between the existence of qualia and physicalism.

Perhaps the clearest version of this argument is Jackson’s knowledge argument. (There are also a number of other arguments in this area — for a very good recent discussion, see Chalmers 1996). This argument asks us to imagine Mary, a famous neuroscientist confined to a black and white room. Mary is forced to learn about the world via black and white television and computers. However, despite these hardships Mary learns (and therefore knows) all that physical theory can teach her. Now, if physicalism were true, it is plausible to suppose that Mary knows everything about the world. And yet — and here is Jackson’s point — it seems she does not know everything. For, upon being released into the world of color, it will become obvious that, inside her room, she did not know what it is like for both herself and others to see colors — that is, she did not know about the qualia instantiated by particular experiences of seeing colors. Following Jackson (1986), we may summarize the argument as follows:

P1. Mary (before her release) knows everything physical there is to know about other people.
P2. Mary (before her release) does not know everything there is to know about other people (because she learns something about them on being released).

Conclusion. There are truths about other people (and herself) that escape the physicalist story.

Clearly this conclusion entails that physicalism is false: for if there are truths which escape the physicalist story how can everything supervene on the physical. So a physicalist must either reject a premise or show that the premises don’t entail the conclusion.

There are many possible responses to this argument, but here I will briefly mention only three. The first is the ability hypothesis due to Lawrence Nemerow (1988) and developed and defended by David Lewis (1994). The ability hypothesis follows Ryle (1949) in drawing a sharp distinction between propositional knowledge or knowledge-that (such as ‘Mary knows that snow is white’) and knowledge-how (such as ‘Mary knows how to ride a bike’), and then suggests that all Mary gains is the latter. On the other hand, P2 would only be true if Mary gained propositional knowledge.

A second response appeals to the distinction between a priori and a posteriori physicalism. As we saw above, the crucial claim of a posteriori physicalism is that (4) — i.e. the claim that S entails S* — is a posteriori. Since (4) is a posteriori, you would need certain experience to know it. But, it is argued, Mary has not had (and cannot have) the relevant experience. Hence she does not know (4). On the other hand, the mere fact that Mary has not had (and cannot have) the experience to know (4) does not remove the possibility that (4) is true. Hence a posteriori physicalism can avoid the knowledge argument. (It is an interesting question which premise of the knowledge argument is being attacked by this response. The answer depends on whether (4) is physical or not: if (4) is physical, then the response attacks P1. But if (4) is not physical, the response is that the argument is invalid.).

A third response is to distinguish between various conceptions of the physical. We saw above that potentially the class of properties defined by the theory-conception of the physical was distinct from the class of properties defined by the object-conception. But that suggests that the first premise of the argument is open to interpretation in either of two ways. On the other hand, Jackson’s thought experiment only seems to support the premise if it is interpreted in the one way, since Mary learns by learning all that physical theory can teach her. But leaves open the possibility that one might appeal to the object-conception of the physical to define a version of physicalism which evades the knowledge argument.

One of the most lively areas of philosophy of mind concerns the issue of which if any of these responses to the knowledge argument will be successful. (See the papers in Ludlow, Nagasawa, and Stoljar 2004.) The ability response raises questions about whether know-how is genuinely non-propositional (cf. Lycan 1996, Loar 1997 and Stanley and Williamson 2001), and about whether it gets the facts right to begin with (Braddon Mitchell and Jackson 1996). As against a posteriori physicalism, it has been argued both that it rests on a mistaken approach to the necessary a posteriori (Chalmers 1996, 1999, Jackson 1998), and that the promise of the idea is chimerical anyway (cf. Stoljar 2000). The third response raises questions about the distinction between the object and the theory conception of the physical and associated issues about dispositional and categorical properties (cf. Chalmers 1996, Lockwood 1992, and Stoljar 2000, 2001.)

14. The Case Against Physicalism II: Meaning and Intentionality
Philosophers of mind often divide the problems of physicalism into two: first, there are the problems of qualia, typified by the knowledge argument; second, there are problems of intentionality. The intentionality of mental states is their aboutness, their capacity to represent the world as being a certain way. One does not simply think, one thinks of (or about) Vienna; similarly, one does not simply believe, one believes that snow is white. Just as in the case of qualia, some of the puzzles of intentionality derive from facts internal to the notion, and from the relation of this notion to the others such as rationality, inference and language. But others derive from the fact that it seems difficult to square the fact that mental states have intentionality with physicalism. There are a number of ways of developing this criticism but much recent work has concentrated on a certain line of argument that Saul Kripke has found in the work of Wittgenstein (1982).

Kripke’s argument is best approached by first considering what is often called a dispositional theory of linguistic meaning. According to the dispositional theory, a word means what it does — for example, the word ‘red’ means red — because speakers of the word are disposed to apply to word to red things. Now, for a number of reasons, this sort of theory has been very popular among physicalists. First, the concept of a disposition at issue here is clearly a concept that is compatible with physicalism. After all, the mere fact that vases are fragile and sugar cubes are soluble (both are classic examples of dispositional properties) does not cause a problem for physicalism, so why should the idea that human beings have similar dispositional properties? Second, it seems possible to develop the dispositional theory of linguistic meaning so that it might apply also to intentionality. According to a dispositional theory of intentionality, a mental concept would mean what it does because thinkers are disposed to employ the concept in thought in a certain way. So a dispositional theory seems to hold out the best promise of a theory of intentionality that is compatible with physicalism.

Kripke’s argument is designed to destroy that promise. (In fact, Kripke’s argument is designed to destroy considerably more than this: the conclusion of his argument is a paradoxical one to the effect that there can be no such a thing as a word’s having a meaning. However, we will concentrate on the aspects of the argument that bear on physicalism.) In essence his argument is this. Imagine a situation in which (a) the dispositional theory is true; (b) the word ‘red’ means red for a speaker S; and yet (c) the speaker misapplies the word — for example, S is looking at a white thing through rose-tinted spectacles and calls it red. Now, in that situation, it would seem that S is disposed to apply ‘red’ to things which are (not merely red but) either-red-or-white-but-seen-through-rose-tinted-spectacles. But then, by the theory, the word ‘red’ means (not red but) either-red-or-white-as-seen-through-rose-tinted-spectacles. But that contradicts our initial claim (b), that ‘red’ means red. In other words, the dispositional theory, when combined with a true claim about the meaning of word, plus a truism about meaning — that people can misapply meaningful words — leads to a contradiction and is therefore false.

How might a physicalist respond to Kripke’s argument? As with the knowledge argument, there are many responses but here I will mention only two. The first response is to insist that Kripke’s argument neglects the distinction between a priori and a posteriori physicalism. Kripke often does say that according to the dispositionalist, one should be able to ‘read off’ truths about meaning from truths a physicalist can reject. (For a proposal like this, see Horwich 2000.) However, the problem with this proposal is, as we have seen, that its background account of the necessary a posteriori is very controversial. As we saw, a posteriori physicalists are committed to what we called the non-derived view about necessary a posteriori truths. But the non-derived view has come under strident attack in recent times.

The second response is to defend the dispositional theory against Kripke’s argument. One way to do this is to argue that Kripke’s argument only works against a very simple dispositionalism, and that a more complicated version of such a theory would avoid these problems. (For a proposal along these lines, see Fodor 1992 and the discussion in Braddon-Mitchell and Jackson 1996). A different proposal is to argue that Kripke’s argument underestimates the complexity in the notion of a disposition. The mere fact that in certain circumstances someone would apply ‘red’ to white things does not mean that they are disposed to apply red to white things — after all, the mere fact that in certain circumstances something would burn does not mean that it is flammable in the ordinary sense. (For a proposal along these lines see Hohwy 1998, and Heil and Martin 1998)

As with the knowledge argument, the issues surrounding Kripke’s argument are very much wide open. But it is important to note that most philosophers don’t consider the issues of intentionality as seriously as the issue of qualia when it comes to physicalism. In different vocabularies, for example, both Block (1995) and Chalmers (1996) distinguish between the intentional aspects of the mind or consciousness, and the phenomenal aspects or qualia, and suggest that it is really the latter that is the central issue. As Chalmers notes (1996; p. 24), echoing Chomsky’s famous distinction, the intentionality issue is a problem, but the qualia issue is a mystery.

The Case Against Physicalism III: Methodological Issues

The final argument I will consider against physicalism is of a more methodological nature. It is sometimes suggested, not that physicalism is false, but that the entire ‘project of physicalism’ — the project in philosophy of mind of debating whether physicalism is true, and trying to establish or disprove its truth by philosophical argument — is misguided. This sort of argument has been mounted by a number of writers, but perhaps its most vocal advocate has been Noam Chomsky (2000; see also Searle 1992, 1999).

It is easiest to state Chomsky’s criticism by beginning with two points about methodological naturalism. In general it seems rational to agree with the methodological naturalists that the best hope for a theoretical understanding of the world is by pursuing the methods which are typical of the sciences. It would then seem rational as a special case that our best hope for a theoretical understanding of consciousness or experience is by pursuing the methods of the sciences — by pursuing, as we might put it, the naturalistic project with respect to consciousness. So Chomsky’s first point is that it is rational to pursue the naturalistic project with respect to consciousness.

Chomsky’s second point is that the physicalist project in philosophy of mind is on the face of it rather different from the naturalistic project. In the first place, the physicalist project is, as we have noted, usually thought of a piece of metaphysics. But there is nothing metaphysical about the naturalistic project, it simply raises questions about what we can hope to explain. In the second place, the physicalist project is normally thought of as being amenable to philosophical argument, whereas it is completely unclear where philosophical argument would enter the naturalistic project. In short, there doesn’t seem anything particularly ‘philosophical’ about the naturalistic project — it simply applies the methods of science to consciousness. But the physicalist project is central to analytic philosophy.

It is precisely at the place where the physicalist project departs from the naturalistic project that Chomsky’s criticism begins to take shape. For insofar as it is different from the naturalistic project, there are a number of ways in which the physicalist project is questionable. First, it is hard to see what the project might be — it is true that throughout the history of philosophy and science one encounters suggestions that one might find out about the world in ways that are distinct from the ones used in the sciences, but these suggestions have always been rather obscure. Second, it is hard see how this sort of project could recommend itself to physicalists themselves — such a project seems to be a departure from methodological naturalism but most physicalists endorse methodological naturalism as a matter of fact. On the other hand, if the physicalist project does not depart from the naturalistic project, then the usual ways of talking and thinking about that project are highly misleading. For example, it is misleading to speak of it as a piece of metaphysics as opposed to a piece of ordinary science.

In sum, Chomsky’s criticism is best understood as a kind of dilemma. The physicalist project is either identical to the naturalistic project or it is not. If it is identical, then the language and concepts that shape the project are potentially extremely misleading; but if it is not identical, then there are a number of ways in which it is illegitimate.

How is one to respond to this criticism? In my view, the strongest answer to Chomsky accepts the first horn of his dilemma and suggests that what philosophers of mind are really concerned with is the naturalistic project. Now, of course, what concerns them is not the details of the project — that would not distinguish them from working scientists. Rather they are concerned with what the potential limits of the project are.

This is a theme which has reached its best expression in the work of Thomas Nagel (1980, 1984, 1999) and allied work by Bernard Williams (1984). According to them, any form of scientific inquiry will at least be objective, or will result in an objective picture of the world. On the other hand, we have a number of arguments — the most prominent being the knowledge argument — which plausibly show that there is no place for experience or qualia in a world that is described in purely objective terms. If Nagel and Williams are right that any form of scientific inquiry will yield a description of the world in objective terms, the knowledge argument is nothing less than a negative argument to the effect that the naturalistic project with respect to consciousness will not succeed.

If what is at issue is the limits of the naturalist project, why is the debate so often construed as a metaphysical debate rather than a debate about the limits of inquiry? In answer to this question, we need to sharply divorce the background metaphysical framework within which the problems of philosophy of mind find their expression, and the problems themselves. Physicalism is the background metaphysical assumption against which the problems of philosophy of mind are posed and discussed. Given that assumption, the question of the limits of the naturalistic project just is the question of whether there can be experience in a world that is totally physical. Nevertheless, when properly understood, the problems that philosophers of mind are interested in are not with the framework themselves, and to that extent are not metaphysical. Thus, the common phrase ‘metaphysics of mind’ is misleading.

The Case for Physicalism

Having considered one side of the truth question, I will now turn to the other: what reason is there for believing that physicalism is true?

The first thing to say when considering the truth of physicalism is that we live in an overwhelmingly physicalist or materialist intellectual culture. The result is that, as things currently stand, the standards of argumentation required to persuade someone of the truth of physicalism are much lower than the standards required to persuade someone of its negation. (The point here is a perfectly general one: if you already believe or want something to be true, you are likely to accept fairly low standards of argumentation for its truth.)

However, while it might be difficult to assess dispassionately the arguments for or against physicalism, this is still something we should endeavor to do. Here I will review two arguments that are commonly thought to establish the truth of physicalism. What unites the arguments is that each takes something from the physicalist world-picture which we considered previously and tries to establish the metaphysical claim that everything supervenes on the physical.

The first argument is (what I will call) The Argument from Causal Closure. The first premise of this argument is the thesis of the Causal Closure of the Physical — that is, the thesis that every event which has a cause has a physical cause. The second premise is that mental events cause physical events — for example we normally think that events such as wanting to raise your arm (a mental event) cause events such as the raising of your arm (a physical event). The third premise of the argument is a principle of causation that is often called the exclusion principle (Kim 1993, Yablo 1992). The correct formulation of the exclusion principle is a matter of some controversy but a formulation that is both simple and plausible is the following:

Exclusion Principle
If an event e causes event e*, then there is no event e# such that e# is non-supervenient on e and e# causes e*.
The conclusion of the argument is the mental events are supervenient on physical events, or more briefly that physicalism is true. For of course, if the thesis of Causal Closure is true then behavioral events have physical causes, and if mental events also cause behavioral events, then they must supervene on the physical if the exclusion principle is true.

The Argument from Causal Closure is perhaps the dominant argument for physicalism in the literature today. But it is somewhat unclear whether it is successful. One response for the anti-physicalist is to reject the second premise and to adopt a version of what is called epiphenomenalism, the view that mental events are caused by, and yet do not cause, physical events. The argument against this position is usually epistemological: if pains don’t cause pain behavior how can it be that your telling me that you are in pain gives me any reason for supposing you are? It might seem that epiphenomenalists are in trouble here, but as a number of recent philosophers have argued, the issues here are very far from being settled (Chalmers 1996, Hyslop 1999). The crucial point is that the causal theory of evidence is open to serious counterexamples so it is unclear that it can be used against epiphenomenalism effectively.

A different sort of response is to reject the causal principles on which the argument is based. As against the exclusion principle, for example, it is often pointed out that certain events are overdetermined. The classic example is the firing squad: both the firing by soldier A and by soldier B caused the prisoner’s death but since these are distinct firings, the exclusion principle is false. However, while this line of response is suggestive, it is in fact rather limited. It is true that the case of the firing squad represents an exception to the exclusion principle — an exception that the principle must be emended to accommodate. But is difficult to believe that it represents an exception that can be widespread. A more searching response is to reject the very idea of causal closure on the grounds, perhaps, that (as Bertrand Russell (1917) famously argued) causation plays no role in a mature portrayal of the world. Once again, however, the promise of this response is more imagined than real. While it is true that many sciences do not exp