Only Consciousness is Physical
As formulated by Descartes and others, mechanism is the doctrine that the objects of sense (“material bodies”) are characterized by extension, solidity, and contact causation. Specifically, the causation involved does not require divine intervention (“occasionalism”) or a scholastic notion of the transfer of forms or Aristotelian teleology (“final causes”) or spooky action at a distance (Newton’s “occult” force of gravity operating across empty space). For mechanism causation works purely by impact and collision, as one moving object touches another and thereby imparts motion to it: extended solid objects come into contact with each other and thus interact causally. The causation is “mechanical”.
With mechanism so understood we can now define physicalism as the doctrine that everything in nature is subject to mechanism—in particular, the mind is subject to mechanism. Everything mental is extended, solid, and causally mechanical. The science of mechanics thus suffices to provide a complete description of reality. This doctrine stands opposed to traditional Aristotelian and scholastic doctrines, as well as doctrines that put God and agency at the center of nature. No occult powers, no spiritual forces, and no teleological processes—just clashing particles in the void. If it were to turn out that we had to recognize such non-mechincal things, then mechanism would be false for material bodies, and so physicalism would also be false.
It is now generally accepted that mechanism as so understood is false of material bodies. There are questions about the extension and solidity components of the doctrine in the light of modern physics, but the causation component is the one that has come under the most serious attack–because of gravitational action at a distance, electromagnetic fields, and causation at the quantum level. To put it simply, causation does not in fact require actual contact between the interacting bodies (or their surfaces). The billiard ball model of the causal structure of the universe has been shown untenable. So the world of bodies has been shown to be non-mechanical and non-physical (in the sense defined). One reaction to this is that, since there is no other well-defined notion of physicalism than that supplied by mechanism, the doctrine of physicalism has been shown to be false. The only clear definition of “physicalism” has turned out to be empirically false, so the doctrine of physicalism has no formulation under which it is true. I accept that conclusion: physicalism is not true of the physical world, i.e. the objects of sense. My question is whether physicalism in the sense defined might yet be true of the mind: granted it is not true of the physical, might it still be true of the mental?
The question may seem absurd or obviously answered in the negative. But not so fast: it is true that it is not easy to show that mental entities are extended and solid without making a prior commitment to a universal mechanism, but these concepts are arguably not essential to the mechanistic world-view. What if it had turned out (as some claim to be actually the case) that bodies are not definable in terms of extension and solidity—that the so-called physical world is not constituted by precisely bounded geometrical objects that are solid and impenetrable? What if it is fields of force all the way down, fuzzy and wispy, potentialities not hard nuggets of matter? Must we then give up mechanism? Not necessarily, because the causation might still conform to the mechanist’s picture: it might consist of things touching (possibly just fields) without any remote causation or teleological causation or transfer of forms. The causal relation might be fully mechanical, even though the ontology departs from the Cartesian paradigm. So we can accept that mental entities are not extended and solid and still ask whether mental causation is a species of contact causation. If so, the mind is causally mechanical; and that will be sufficient to establish a mechanistic view of how the mind works. Then we can say that the mind is physical in the sense that it works mechanically: all we need is mental causation by means of contact causation. It doesn’t work by divine intervention or teleological causation or spooky action at a distance. If that is so, then the mind, including consciousness, can be rightly characterized as “physical”, since that notion is to be understood in terms of the nature of the causation involved, i.e. whether the causation is mechanical.
But now the tricky question is whether mental causes and effects make contact. If we accept that mental entities are not in space and have no boundary or surface, then it is indeed difficult to see how they can make contact in the way bodies make contact (or fail to given the facts of physics). Can the occupants of the mind toucheach other? But what is the significance of that notion in relation to ordinary physical causation? It doesn’t logically presuppose extension and solidity, or even location in three-dimensional space; what it connotes is the idea of direct unmediated connection—as opposed to final causes, action at a distance, or scholastic forms flitting from one object to another. It is the idea of immediate localized proximate efficient causation—a prior state of affairs directly bringing about a later state of affairs without any outside divine assistance or teleological targeting or crossing of spatial chasms. When Descartes and others spoke of “mechanical” causation that is what they meant to exclude—the conception of causation handed down from Aristotle and the scholastics. But there was no reason to resist that conception for mental causation, since no one had suggested that mental causation operated in such strange ways. Mental causation was clear and intelligible compared to the kinds of causation contemplated for material bodies. How could there be puzzling action at a distance if there is no distance in the mind to start with? How could anyone think that the motion of the mind arises from naturally given ends if the mind didn’t move?
Mental causation operates by something analogous to physical contact between bodies; or perhaps we should say that we model the idea of physical contact on the kind of intimate connection that holds between items in the mind. When a visual experience causes me to make a judgment the causation is direct and unmediated—not acting across empty space or aided by God. It is transparent and intelligible. When a desire causes me to make a choice the connection is plain to see: the desire is tied explicably to the choice. If we think of the mind as a mental substance consisting of parts, then these immaterial parts are in “contact” with each other—they impinge on and influence each other. In particular, we have discovered nothing analogous to action at a distance in the mind—the causation is just old-fashioned proximate causation. So the mechanistic picture of causation is not violated by mental causation, despite the fact that mind and body are distinct. This means that even a Cartesian dualist can accept the mechanistic picture of mental causation—though he must reject mechanism for physical causation, given what we have empirically discovered. It is not as if Newton had also discovered action at a distance in the mind and hence was forced to postulate an occult brand of causation operating in the mind. Mental causation could therefore be conceived according to the mechanical model, suitably tweaked to fit mental ontology. We can even have a Cartesian dualism of substances combined with a view of mental causation that conforms to the mechanical model—as opposed to the causation that actually operates in the physical world. Thus mechanism applies to the mind but not to the body: for causation is “occult” in the latter case but not in the former.
But if the only coherent notion of physicalism is tied to mechanism, then it turns out that physicalism is true of the mind but not the body! The mind is physical (i.e. operates by means of mechanical causation) while the body is not physical (i.e. does not operate by means of mechanical causation). The mind is more of a “machine” than the body. That is, the mind does not work by means of the kinds of unintelligible causation that appear to pervade the physical universe—in particular, gravitational and electromagnetic causation. Descartes’ mechanism of the physical world was ruined by the science of physics, beginning with Newton; but nothing in the science of psychology has ever upended common sense views of mental causation as proximate efficient causation. The core component of classical mechanism was its conception of causation (extension and solidity being optional), but that conception was empirically refuted for the physical world. However, in the case of the mind an essentially similar conception of causation has not been refuted by science, so mechanism holds for mental causation—the mind is machine-like in its mode of causal operation. The mind better fits Cartesian mechanism than the body does. The mind is therefore physical, according to the doctrine of physicalism as defined by reference to mechanical causation. 
Now it is not that I wish to advocate the metaphysical thesis that the mind is physical while the so-called physical world is not—that physicalism is true of consciousness but not of the brain (or the kidneys). My point is just that if we insist on defining a metaphysical doctrine of physicalism in these kinds of terms then that is the consequence we must accept. And it is not that some other definition of physicalism leads to less counterintuitive results, since it is doubtful that there is any other workable definition of physicalism. The right lesson to draw is that the concept of physicalism is too contestable and up for grabs for such metaphysical doctrines to make much sense. Surely it is a reductio ad absurdum of this entire way of thinking that the traditional definitions lead to the conclusion that only consciousness is physical: not brains or hearts or planets or atoms, but thoughts or pains or perceptions. But that is the inevitable result if we persist in trying to apply these doctrines in the traditional style. Physics has made physicalism obsolete with regard to the physical world. We don’t want to find ourselves resurrecting it for the mind. 
 Some have suggested eliminating the concept of causation altogether from physical science, given the demise of mechanism, but such a demand has not been made for psychology—we just don’t have the same reasons for disquiet over causation in the case of psychology. For instance, the problems for causation inherent in quantum theory have no counterpart for psychology. Compared to physics psychology is a causal oasis.
 This is by now a familiar story; my point has been that classical physicalism (the well-defined kind) leads to the conclusion that only the mind is physical—not a happy result. We don’t want to end up saying that immaterial mind is physical while material body is not!