Copyright © 1999 Massachusetts Institute of Technology
The problem of mental causation is the most recent incarnation of the venerable MIND-BODY PROBLEM, Schopenhauer's "world knot." DESCARTES held that mind and body were distinct kinds of entity that interact causally, but waffled over the question how this was possible. Mind-body dualism of the Cartesian sort is no longer popular, but variants of the Cartesian problem remain, and waffling is still in fashion. Current worries about mental causation stem from two sources: (1) "nonreductive" conceptions of the mental; and (2) externalism (or anti- INDIVIDUALISM) about mental "content."
Although most theorists have left dualism behind, many remain wedded to the Cartesian idea that the mental and the physical are fundamentally distinct. Mental properties, though properties of physical systems, are "higher-level" properties not reducible to or identifiable with "lower-level" properties of those systems. Most functionalist accounts of the mind embrace this picture, endorsing the slogan that mental properties are "multiply realizable." Take the property of being in pain. This property, like the property of being an eye, is said to be a functional, "second-order" property, one that a creature possesses by virtue of possessing some first-order "realizer" -- a particular physical configuration or process, for instance. Pains are capable of endless "realizations" in the human nervous system, in the very different nervous system of a cephalopod, and perhaps in silicon-based systems of Alpha Centaurians -- or appropriately programmed computing machines. There is, then, no prospect of locating a unique physical property to identify with pain.
When, however, we try to reconcile "multiple realizability" with the idea that the physical realm is causally self-contained, trouble arises. Consider your body, a complex physical system composed of microparticles interacting in accord with fundamental physical laws. The behavior of those particles, hence the behavior of your body, is completely determined (albeit probabilistically) by those laws. Now suppose you step on a tack, experience a pain, and quickly withdraw your foot. Common sense tells us that your pain played a causal role in your foot's moving. But can this be right? Your experiencing a pain is a matter of your possessing a "higher-level" property, a property thought to be distinct from any of the properties possessed by your "lower-level" physical constituents. It appears, however, that your behavior is entirely determined by "lower-level" nonmental goings-on. In what sense, then, is your experience of pain "causally relevant" to the movement of your foot?
Some philosophers have responded to this difficulty by adopting a deflationary view of CAUSATION. As they see it, causation is just counterfactual dependence: roughly, if E would not have occurred unless C had, then C causes E (LePore and Loewer 1987). Others, appealing to scientific practice, have suggested that we replace metaphysically loaded references to causes with talk of causal explanation (Wilson 1995). Still others argue that the causal relevance of "higher-level" properties requires reduction: "Higher-level" mental properties must be identified with "lower-level" properties (Kim 1989). This strategy resolves one problem of mental causation, but at a cost few philosophers seem willing to pay.
Lack of enthusiasm for reduction is due in part to the widespread belief that mental properties are "multiply realizable," hence distinct from their physical realizers, and in part to a no less widespread commitment to externalism. Externalists hold that the "contents" of states of mind depend on agents' contexts. Wayne, for example, believes that water is wet. Wayne's belief concerns water, and the "content" of his belief is that water is wet. Imagine an exact duplicate of Wayne, Dwayne, who inhabits a distant planet, TWIN EARTH, an exact duplicate of Earth with one important difference: the colorless, tasteless, transparent liquid that fills rivers and bathtubs on Twin Earth differs in its molecular constitution from water. Water is H2O. The substance on Twin Earth that Dwayne and his fellows call "water" is XYZ. Now, so the story goes, although Wayne and Dwayne are alike intrinsically, their thoughts differ. Wayne believes that water is wet; whereas Dwayne's beliefs concern, not water, but what we might call "twin water" (see Putnam 1975).
Thought experiments of this sort have convinced many philosophers that the contents of thoughts depend, at least in part, on thinkers' surroundings and causal histories (see Burge 1986; Davidson 1987; Baker 1987). A contextualism of this sort introduces a new twist on the problem of mental causation, and simultaneously renders REDUCTIONISM even less attractive. Surely the contents of your thoughts are relevant to what those thoughts lead you to do. You flee because you believe the creature on the path in front of you is a skunk. Had you believed instead that the creature was a cat, you would have behaved differently. If the content of your belief -- its being a belief about a skunk -- depends on your causal history, however, how could it make a here-and-now physical difference to the way you move your body?
A molecule, a billiard ball, a planet, or a brain behaves as it does and reacts to incoming stimuli because of its intrinsic physical makeup. But if everything you do is a function of your intrinsic physical properties, and if the contents of your thoughts depend on relations you bear to other things, then it is hard to see how the contents of your thoughts could make any difference at all to what you do.
Again, some philosophers have sought to accommodate externalism and mental causation via deflationary accounts of causation or appeals to explanatory norms. Others have defended a notion of "narrow content," mental content that depends only on agents' intrinsic composition (Fodor 1987). Wayne and Dwayne, for instance, are said to entertain thoughts that have the same "narrow content" but differ in their "broad content." Externalists have been unenthusiastic about "narrow content." And, in any case, even if we embrace "narrow content," so long as we assume that mental properties are irreducible "higher-level" properties of physical systems, we are left with our initial worry about the causal irrelevance of "higher-level" properties.
Externalism aside, perhaps we could make progress by distinguishing predicates and properties. Predicates apply to objects by virtue of properties those objects possess, but not every predicate designates a property. The predicate "is a tree" applies to objects by virtue of their properties, but there is no property of being a tree common to all trees. Perhaps mental predicates are like this. The predicate "pain," for instance, might apply to many different kinds of object, not because these objects share some single property, but because they are similar in important ways: they possess distinct, though similar, first-order physical properties (which have uncontroversial causal roles). A view of this sort allows that "pain" applies truly to creatures in distress, although it obliges us to abandon the philosopher's conceit that the predicate "pain" thereby designates a property shared by every creature to whom "pain" is truly predicable.
Whether these remarks are on the right track, they suggest that the philosophy of mind would benefit from an infusion of good old-fashioned metaphysics. Until we are clear on the nature of properties, for instance, or the character of "multiple realizability," we shall not be in a position to make headway on the problem of mental causation.
Baker, L. R. (1987). Saving Belief. Princeton: Princeton University Press.
Burge, T. (1986). Individualism and psychology. Philosophical Review 45:3-45.
Davidson, D. (1987). Knowing one's own mind. Proceedings and Addresses of the American Philosophical Association 60:441-458.
Fodor, J. (1987). Psychosemantics. Cambridge: MIT Press.
Heil, J., and A. Mele, Eds. (1993). Mental Causation. Oxford: Clarendon Press.
Jackson, F. (1996). Mental causation. Mind 105:377-413.
Kim, J. (1989). The myth of nonreductive materialism. Proceedings and Addresses of the American Philosophical Association 63:31-47. Reprinted in Supervenience and Mind. Cambridge: Cambridge University Press, 1993, pp. 265 - 284.
LePore, E., and B. Loewer. (1987). Mind matters. Journal of Philosophy 84:630-642.
Putnam, H. (1975). The meaning of "meaning." In Mind, Language and Reality. Cambridge: Cambridge University Press, pp. 215-271.
Wilson, R. (1995). Cartesian Psychology and Physical Minds. Cambridge: Cambridge University Press.
Copyright © 1999 Massachusetts Institute of Technology