Why Property Dualists Must Reject Physicalism About Substance
First Draft: July 21, 2009
Consider an ontological scheme that features qualia as irreducible properties of the universe, alongside the usual inventory of sparse physical properties identified by a penultimate physics. We have property dualism: both mental and physical properties are sui generis. And we think we know exactly where in philosophical space to locate such a view -- it is distinct from substance dualism. For many contemporary property dualists are highly critical of substance dualism, claiming instead that while mental and physical properties are distinct, all substances are nonetheless physical. That is, they hold both:
(SP) Substance Physicalism: all substances are physical.
(PD) Property Dualism: mental and physical properties are distinct.
Indeed, Substance Physicalism has been characterized as the default position in philosophy of mind, being “a starting point for discussion rather than a conclusion in need of defense” (Kim, 2006, p. 274). And this is all well and good, says the canon, since contemporary philosophy of mind sees the question of the nature of substance as being settled in favor of the physicalist. Dualism about properties, in contrast, is regarded as a live option. As Jaegwon Kim writes in a recent textbook: “…the fact is that substance dualism has played a very small role in contemporary discussions of philosophy of mind….Dualism is no longer a dualism of two sorts of substances; it is now a dualism of two sorts of properties, mental and physical” (2006, p. 51).
Kim’s statement aptly characterizes the state of play in the field. But today I shall urge that the field is mapping philosophical space incorrectly. According to the leading conceptions of the nature of substance in contemporary metaphysics, the bundle theory and the substratum theory, property dualism cannot comfortably appeal to substance physicalism. Instead, property dualism, when paired with these theories of substance, leads to substance dualism. Further, I offer in principle considerations that seem to extend to any theory of substance. If I am correct then there are not two dualist options here but one. You may reject souls and ectoplasm, but if consciousness is rock bottom for you, so too are mental substances. The space of possible solutions to the mind-body problem is thereby constricted. Today I aim for negative progress.
Here is how I shall proceed. Section 1 urges that insofar as property dualism appeals to a bundle theory of substance it cannot appeal to substance physicalism. Indeed, it seems naturally wedded to substance dualism; yet Section 2 contends that should the property dualist instead turn to the substratum view of substance then the same fate ensues. In the course of discussing these two theories I also identify general considerations intended to apply to any theory of substance. Section 3 then offers a further general consideration, urging that insofar as one posits irreducible mental properties and appeals to the category of substance, physical substances will be unsuitable bearers of such properties. The challenge to the property dualist is to develop a plausible ontology of substance that lends itself to substance physicalism or at least rules out substance dualism. Alternately, insofar as property dualism is still appealing to her – and at this point it may no longer be -- she could embrace substance dualism instead.
1. Substances as Bundles of Universals or Tropes
The property dualist’s universe is populated by properties – the mass of a neutrino, the flavor of a Chianti, the rich hues of a sunset. For present purposes we do not need to decide whether this universe of properties is a universe of repeatables (universals) or abstract particulars (“tropes”). What is important is that the property dualist holds that at least certain mental properties -- paradigmatically phenomenal properties but perhaps intentional ones as well -- are metaphysically basic. Now, let us ask: how are we to understand the relationship between a thing (e.g., the tree) and its properties (e.g., its brownness)? Armstrong says that here, “two different models compete for allegiance of philosophers”, referring to the bundle and substratum views (1989, 59). We shall consider each view in turn, beginning with the bundle theory.
Notice that when we conceive of an object what comes to mind are its features. Indeed, it is difficult to think of what would be left over if we tried to conceive of the object without any of its features. In keeping with this observation, the Bundle Theory holds that a substance is just a bundle of its properties. Of course, not every bundle of properties is an object, so we should ask: what unites bundles that are bona fide substances? Bertrand Russell’s influential suggestion was an unanalyzable relation of compresence - a relation that obtains between any two properties that are properties of the same object (1948, 312). A complex of compresence is a class of properties each of which has the compresence relation to each member. A given particular is a complete complex of compresence – a complex in which no further universals can be added because any further universal would not be compresent with at least one member of the group. A further feature of the theory is worth noting as well: according to a crude version of the bundle theory, all of a substance’s properties are included in the nature of the bundle. But surely this cannot be right; otherwise, substances would not persist over time. So some properties of a given bundle are accidental, while others are essential.
Now let us ask: Is the conjunction of property dualism and the bundle theory even compatible with substance physicalism? Here is the problem: according to the bundle theory, substances are just bundles of the properties that they possess. So why is the mind, which is constituted by irreducible non-physical properties, really a physical substance at all? An example will help illustrate the problem. Suppose you are walking along a beach. As you experience the rich blues of the ocean and sky, and feel the warmth of the sun on your face, why is the mind – i.e., the substance which has the experiential properties -- a physical thing? Why is there not instead a non-physical bundle that instantiates the experiential properties, since, according to the conjunction we are considering, there would be a bundle that has irreducible phenomenal properties as constituents? Of course, if you reject property dualism for the type identity theory, this issue will not be compelling: the same substance has both kinds of features because mental properties just are physical ones. But we are assuming a property dualist conception; and on this conception there is a categorical divide between mental and physical properties, and when substances are bundles, it is important to ask: Given that non-physical properties are constituents of the bundle, why would the bundle be physical? After all, according to property dualism, consciousness is the mark of the mental if anything is. If anything is to characterize the nature of mind, wouldn’t it be phenomenal properties? This point is in fact the very kernel of property dualism: for the property dualist is urging that we need to characterize such properties as basic elements of the universe in order to explain the fundamental nature of mind.
We can now frame the initial challenge: in order to uphold substance physicalism the property dualist must explain why the presence of irreducible qualia do not constitute sui generis mental substances. In the context of property dualism, if not type identity, this question has to be taken seriously.
Could the property dualist claim that there is nonetheless a single “hybrid” substance - one which consists in physical and qualitative properties? Perhaps. But she needs to provide a principled reason to regard this “hybrid” substance as being physical, any more than non-physical, otherwise, she gives up substance physicalism. To do this, she must illustrate why any physical substance can be the bearer of irreducibly mental properties. After all, not every property can be compresent in the same bundle as every other. Consider any property and its negation, or consider properties that cannot be coinstanced as a matter of law (a particle’s both having mass and traveling at the speed of light). It will not do to simply assert, as the naturalistic property dualist currently does, that the brain has non-physical properties. The dialectical burden is on the property dualist to show that a substance that is categorically distinct from the mental realm can nonetheless be the bearer of something in the physical realm. I take this matter up in further detail in Section 3.
Even setting aside the issue of whether any physical substance could be the bearer of irreducible qualia, there is a further reason to suspect that the property dualist’s appeal to substance physicalism is dubious: the property dualist is hard-pressed to maintain that brains, without qualia, are also minds. (1) Question: what does God need to do to ensure that our world has physical bundles? Answer: God must specify the mosaic of sparse physical properties, the compresence relation and (perhaps) spacetime. Next question: but what does God need to do to make it the case that our world has minds? Property dualist answer: God must make it the case that the world has irreducible qualia. But no genuine substance physicalist can venture this answer – if God needs to add mental properties to the world to create minds, minds are surely not physical substances.
(2) Property dualism holds that mental properties nomologically supervene on physical properties. But property dualism rejects supervenience in worlds that are physical duplicates of ours where our psychophysical laws fail to hold. For consider zombie worlds: such contain systems that are “…physically identical to a conscious being, but that lacks at least some of that being's conscious states” (Chalmers, 2002). Zombies have brains, but do zombies have minds? I doubt the property dualist will want to say this. Again, consciousness is the mark of the mental. Now consider: Brains have different modal properties than minds do -- brains can exist without phenomenal properties, not so with minds. Brains do not have phenomenal consciousness essentially; minds plausibly do. But if minds and brains differ in this way, surely they cannot be identical.
(1) and (2) are not just arguments against the conjunction of property dualism and the bundle theory, mutatis mutandis, they are general arguments that claim that property dualism cannot be genuinely physicalistic about substance. They also suggest that property dualism is, even in its current naturalistic form, at rock bottom a substance dualist position.
Just as property dualism forms an unnatural marriage with substance physicalism, we have observed that it seems to fit all too well with substance dualism. Allow me to examine this natural fit in a bit more detail; I’d like to sketch the barest outline of a bundle version of a “Naturalistic Substance Dualism” (if you will). Let us ask: How would the mental and physical substances relate? The Naturalistic Dualist holds that to explain consciousness one must posit basic psychophysical laws. And as per the bundle theory, the mental and physical properties figuring in the laws are constituents of the mental and physical bundles, respectively. This leaves us with nomologically necessary relations of dependency between the mind and the brain. That is, corresponding to each non-physical mind there is a brain, a complex physical substance ultimately made up of fundamental microphysical entities (e.g., strings). The presence of the brain – that is, the presence of a physical substance in a highly sophisticated neural configuration -- is nomologically sufficient for a mind.
It is easy in the current physicalistic climate to dismiss a view when one suspects it to be a substance dualist position. But we should ask: Is Naturalistic Substance Dualism (NSD) really so implausible? Traditionally, the chief drawback of substance dualism is that it has an inadequate account of mental causation. If the mind is non-physical, it lacks a position in physical space. So how can it interact with physical substances? But upon reflection, with NSD psychophysical causation is no more problematic than it was under property dualism proper. (Caveat: Mental causation is highly problematic for property dualism. Qualia look to be epiphenomenal. I am merely saying NSD doesn’t add any additional problems with mental causation to the mix). Here is why. Assuming that Naturalistic Substance Dualism does not introduce a form of substance causation – that is, assuming it maintains that causation is a relation among events (where events are construed as property instances) -- the problem of how mental substances enter into the causal nexus is upon reflection just the standard problem of how mental properties can be genuinely causal given that all the real causal work appears to be done in the microphysical realm. Now, the naturalistic property dualist currently rejects the view that phenomenal states are non-spatiotemporal; instead, they generally say that phenomenal properties are instantiated where the brain is. So the mind (i.e., the non-physical bundle), having phenomenal state tokenings as constituents, while being distinct from the brain, is itself spatiotemporal, coinciding with the brain. You may dislike NSD for other reasons, but as far as I can tell, there is no new problem of mental causation here.
I can offer one further consolation to the property dualist as well: There is a sense in which in the context of the bundle theory substance dualism is only as ontologically costly as property dualism itself. Substances are not basic--they are just metaphysical constructs of universals or tropes.
I have tried to make the marriage more palatable, but like any forced marriage, this will strike the unwilling fiance’ with distaste. So let’s consider whether there are any other ways that the property dualist could respond to my initial challenge. Could she say that phenomenal properties are not constitutive of any bundle, being instead accidental properties of physical substances (e.g., brains)? It is difficult to see how the property dualist, of all people, can say this with a straight face: property dualism says that such properties are ontologically fundamental, alongside the sparse physical properties. The sparse physical properties type individuate the fundamental particles; yet on this view, qualia do not type individuate anything, being merely accidental properties of the mind/brain. Phenomenal properties get lofty credentials, but no office.
This is not to assume that every property is type individuative; it is just to say that it is odd that the property dualist is saying that mental properties are ontologically basic – otherwise the mind is not explainable – yet they do not type individuate the mind (i.e., the mind/brain). Only physical properties do. If this is the case then zombies should have minds. And God shouldn’t have to do anything extra to brains to add mentality to the world. But the property dualist would reject these claims. So: Where is the argument for the view that phenomenal properties are not type individuative?
To sum up: the property dualist should not appeal to the bundle theory if she would like to be a physicalist about substance. For on this view, why wouldn’t phenomenal properties be constituents of the bundle? And if they do constitute the bundle, then why is the bundle physical? I’ve also offered general considerations suggesting that property dualism must reject substance physicalism and that further, it seems to naturally lend itself to substance dualism. These considerations do not rely on a specific view of substances.
II. Substance as Propertied Substrata
It is instructive to see why one would turn away from substances as bundles to the substratum view. Consider a bundle theory of universals. It has the following unintuitive consequence: according to the Identity of Indiscernibles, if a and b have all and only the same universals they are the very same object. But consider two fundamental particles having all and only the same universals. According to the bundle theory, they will be the very same object. Of course if one is a genuine bundle theorist she will bite the bullet, for the bundle view is saying that substances are metaphysically composed of all and only properties, together with a bundling relation, so if her properties and relations are repeatables, so too are her objects. So the objection only goes so far, crystallizing the real commitment of the bundle theorist. Unpalatable to some, the idea is that substances are a species of repeatable.
But this objection does not apply to the trope version of the bundle theory. Still, both the trope and universals version of the theory face the following worry: bearing in mind that not any collection of properties is an object, why would adding a relation like compresence, which is just another universal or trope, turn the collection of universals or tropes into a particular? (Martin, 1980). A consideration like this will not bother the bundle theorist, but it is decisive for some.
Thus bare substrata are invited onto the scene. According to the substratum theory a substance is a bundle of properties, together with its substratum. Substrata are summoned to play two crucial roles. (i), The properties are instantiated by the substratum and in this way are bound together into a single particular. So we now have genuine particulars. And (ii), by virtue of the uniqueness of each substratum, substrata serve to distinguish indiscernible objects, a virtue for those who appeal to universals.
Yet substrata are obscure, being, as Locke himself remarked, “something I know not what. We cannot sense them directly -- if they exist they are always united with properties. D.M. Armstrong, who holds this view, suggests that a substratum (which he calls a “thin particular”) can be thought of separately, in abstraction from the thing’s properties. I take it that he means that we can conceptualize a metaphysical category that plays roles (i) and (ii) while substrata themselves are always bound to properties in the world. Yet despite this obscurity, advocates of the substratum view contend that substrata are necessary to understand particularity. Maybe so.
In any case, let us suppose that property dualism joins forces with the substratum view. Now, given the considerations raised in the context of the bundle theory, let us ask: why is a substance tokening phenomenal properties supposed to be physical, rather than non-physical? Our earlier problem seems to arise: It looks as if there is, on the one hand, a mental substance, and on the other hand, a physical one. Or perhaps there is a single “hybrid” one, in which case the substance would not be entirely physical.
Can the property dualist deny that the mental properties individuate any substance, saying instead that they are accidental features of the brain? This move seemed unmotivated in the context of the bundle theory. However, the property dualist now has an additional dialectical move at her disposal: she could urge that only physical properties individuate the substance (together with the substratum) because the substratum itself is physical. This move is controversial. For instance, C.B. Martin observes that “… substrata qua substrata do not and cannot divide into kinds at all. If a set of properties is specified making up a kind and it is attributed to a substratum, then the resultant is an object of a kind” (Martin, 1980, 7). But let us pursue it nonetheless.
We’ve already observed that substrata are mysterious. Can one say more about their nature above and beyond that they serve to individuate the object and are the bearers of properties? One cannot say much; but Armstrong at least distinguishes two ways of saying very little: (i), Strong Haecceitism ". . . holds that a and b each have a unique inner essence, a metaphysical signature tune as it were, something apart from their repeatable properties…which distinguishes them (1989, p. 59). (ii), "Weak haecceitism", in contrast, denies that substrata have inner essences of this sort; instead, substrata differ solo numero. Armstrong writes, “There is certainly no call to think of haecceity as a unique inner nature or essence possessed by each particular, something property-like, although a property necessarily limited to one thing…When we have said that different particulars are numerically different, then we appear to have said all that can be said about the nature of particularity” (1997, p. 108).
Would either (i) or (ii) deliver substance physicalism to the property dualist, supporting a position in which the substrata themselves are physical? Let us consider each option in turn, beginning with the second. (ii), If substrata differ solo numero then it seems that to the extent that objects can have a physical or non-physical character at all it is determined by what properties they possess. Otherwise the substrata would not merely differ numerically. Then, as before, irreducible qualia would seem to call for an ontological commitment to mental substances.
(i), If substrata have unique inner natures it is also difficult to say how the substratum itself (rather than the properties or substance as a whole) can be physical or non-physical to begin with. For a substratum’s being physical or non-physical would seem to involve its having features which different substrata can share, and substrata natures are not supposed to be properties. Is there some other sense in which substrata could have unique natures that are physical? One cannot merely assert they are physical because they are spatiotemporal – various putative non-physical substances (e.g., ghosts and more soberly, Lowe’s persons) have been said to nonetheless exist in space and time. Another sense in which something is said to be physical is when it is named in the vocabulary of a current or future physics itself. But physics does not talk of substrata or haecceities. Although if substrata exist physicists may unwittingly refer to such, we cannot look to physics for an identification of these entities as being within the domain of the physical the way that we can for an identification of what the sparse physical properties are. It is thereby difficult to avoid seeing a substratum as physical or non-physical only in a derivative sense. Substrata are such by virtue of the properties they instantiate. Alas, it appears that Martin’s suspicion was right on target. And this brings us full circle to the puzzle we began with: why are substances instantiating qualitative properties physical?
I have thus far mainly raised this problem in the context of two particular theories of substance. I shall now issue a more general challenge.
III. Leibniz’ Machine
Is physical substance suitable to be the bearer of experiential properties? Consider Leibniz’ reflection with respect to perception:
17. It must be confessed, moreover, that perception and that which depends on it, are inexplicable by mechanical causes, that is by figures and motions. And supposing there were a machine so constructed as to think, feel and have perception, we could conceive of it as enlarged and yet preserving the same proportions, so that we might enter it as into a mill. And this granted, we should only find on visiting it, pieces which push one against another, but never anything by which to explain a perception (1951, p. 536).
We can similarly ask: can a thing be conscious by virtue of the physical interaction of its parts? Your experience of the blueness of sky is itself not found in any neuron or minicolumn. So why is physical substance the appropriate bearer for experiential properties?
This may seem to be a sophomoric question by today’s standards: consciousness is a higher-level physical property of complex systems, and is not found in any neuron or microcircuit alone. For instance, Dennett, Baars, Daheane, myself and others claim that the physical basis of consciousness appears to be a network of pancortical connections that direct contents, when a suitable threshold is reached, into a “global workspace” (cite). A genuine physicalist can say that consciousness is not itself in any particular neuron, but is a higher-level physical property of physical elements of the brain in suitable configuration. Indeed, she can even bite the bullet and say consciousness is found in the neuron, (e.g., in the microtubules) or minicolumn. What is important is that she hold:
(CP): Consciousness is entirely determined by the sparse physical properties of the brain, together with facts about how the sparse properties are arranged.
Qualia are real properties of the universe, they are not properties “over and above” properties discovered by a completed physics, together with their spatiotemporal arrangement.
But the property dualist cannot answer our question in this manner. On her view, consciousness is not identical with a higher-level physical property of any sort. For consider David Chalmers’ remarks about Mary, the neuroscientist. What explains that Mary has a new experience when she exits the room and sees a red tomato for the first time?
Indeed, nobody knows why these physical processes are accompanied by conscious experience at all. Why is it that when our brains process light of a certain wavelength, we have an experience of deep purple? Why do we have any experience at all? Could not an unconscious automaton have performed the same tasks just as well? (1998, p. 91)
An explanation of the physical workings of the brain alone will not suffice to explain Mary’s experience. So how is consciousness supposed to arise from neural activity? Chalmers reflects: “I am not denying that consciousness arises from the brain. We know, for example, that the subjective experience of vision is closely linked to processes in the visual cortex. It is the link itself that perplexes, however.” (2002, p. 93) The missing link, Chalmers’ suggests, are the fundamental psychophysical laws specifying how experience depends on physical processes. “It is this bridge that will cross the explanatory gap” (2002, 96). In a passage that reminds one of Leibniz’ machine Chalmers explains the gist of his ‘explanatory argument’, an argument which he regards as a central reason to adopt property dualism:
The trouble is that physical theories are best suited to explaining why systems have a certain physical structure and how they perform various functions. Most problems in science have this form; to explain life, for example, we need to describe how a physical system can reproduce, adapt and metabolize. But consciousness is a different sort of problem entirely, as it goes beyond the scientific explanation of structure and function (2002, 96).
So we are back inside Liebniz’ Mill. And now consider: Why, given this position, would any physical substance be suitable as a bearer of experiential properties? If one looks to the workings of the brain one will only find computational or physical principles that will explain the biological basis of awareness; one cannot find a theory of phenomenal consciousness. The physical world is simply the wrong place to look.
Plantinga saw this all along. He was convinced that such a problem plagued physicalistic accounts of intentionality. He writes:
It’s a little like trying to understand what it would be for the number seven, e.g., to weigh 5 pounds, or for an elephant (or the unit set of an elephant) to be a proposition.…We can’t see how that could happen; more exactly, what we can see is that it couldn’t happen. A number just isn’t the sort of thing that can have weight; there is no way in which that number or any other number could weigh anything at all (2006, p. 21).
Of course, Descartes famously held that bodies were unsuitable bearers of mental properties because the mechanical activity of the body could not be the basis for intelligent activity. In this day and age some may find it is difficult to appreciate Descartes’ perspective; we may think that cognitive science will explain human intelligence or that artificial intelligence is technologically possible, for instance. Even certain of Descartes’ own contemporaries questioned his claim that only an immaterial substance could think. Still, we can appreciate that if it were the case that the brain couldn’t give rise to thought then material substance would be an unsuitable bearer of mental properties. For in that case, thought would not be not part of the mechanical world. Mechanical stuff would not be the right sort of thing to instantiate thought.
Mutatis mutandis, if you suspect that materialism lacks suitable resources to answer the hard problem, and if you suspect property dualism has them, then Plantinga’s observations apply. The mental and physical realms are, by the property dualists own admission, categorically distinct. Property dualists claim that a walk through the mill will only reveal fundamental microphysical entities (e.g., loops, fields) and physical properties. They can’t find consciousness in the mill. Stronger yet: they see that consciousness cannot be found in the mill. Brains are the wrong sort of substance in which to seek an answer to the hard problem. So shouldn’t the property dualist also see that physical substance can not be the bearer of qualia?
It is here that the property dualist may outdistance Descartes. The bare assertion that one can imagine that one’s mind exist without one’s body is unsatisfying: prima facie, one may claim to be imagining this while being necessarily physical. (Hill and McLaughlin 1999. McLaughlin, 2007). In the eyes of many, the ascent from bare assertion to metaphysical possibility has proved to be a treacherous one. Consider then a different route to substance dualism -- one which takes irreducible qualia as a point of departure. Don’t sui generis mental properties call for sui generis bearers?
So here is where we have arrived: Tim Crane writes, “… the mind-body problem is therefore posed by the question of how to articulate an adequate alternative to the Cartesian view, given that we must reject its commitment to two substances” (2003). It is no longer clear that the property dualist should share this commonly held conception of the state of play in philosophy of mind. Instead, she may now see the challenge as being either to make substance dualism palatable, Cartesian or otherwise, or to reject property dualism altogether.
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. Thanks to Jaegwon Kim and Jose Bermudez for helpful discussion, and to Mark Bickhard, Michael Heumer, John Heil and Jerry Vision for their informative written comments and discussions. Thanks to the NEH for financial support.
. By “substance” I do not mean “substratum” but the entire object, broadly construed to include both physical and non-physical substances. Some use the two expressions interchangeably.
. Like many, I believe the strongest case for property dualism stems from the phenomenon of consciousness. Chalmers’ case for property dualism is very influential; I shall make heavy use of his view. For an interesting recent case for dualism involving intentional properties see Plantinga, “Against Materialism.” Maybe a dualism about one would amount to a dualism about the other – i.e., some have questioned the common view that intentional and phenomenal states are truly separable (Graham, G., Horgan, T., and Tienson, 2007).
. Descartes famously claimed that immaterial substances were characterized by thought, whereas physical substances were characterized by extension. Many reject Descartes’ bifurcation; as Princess Elizabeth asked: why can’t a physical substance think? Section 3 will take a different approach than Descartes; I am not claiming that only immaterial substances can instantiate thought. Herein, I leave it open whether thought is a physical property. What gives me pause, however, is the contemporary property dualist’s view that consciousness is sui generis, yet a physical substance is its bearer.
. This is a kind of “emergent substance dualism”.
. I do not mean to suggest that this notion of such a mental substance is unproblematic.
. Some substance dualists take there to be only a single physical substance (e.g., spacetime). To keep things simple, I’ll speak of the physical substance in question as being the brain. Of course if the mind is supposed to be extended then the physical substance in question goes beyond the brain.
. See O'Leary-Hawthorne, J., 1995.
. Note: On this view substrata, not the object directly, instantiate properties.
. See Locke, 1960,II.xxii.6; see also II.xxiii.2.
. This section is inspired by Plantinga’s “Against Materialism” where he issues a similar challenge to the materialist concerning intentional properties.
. See also Chalmers, 1997, p. 106. Kim too now holds qualia defy functional explanation (2005).
. Descartes, 1986.