SWIF Philosophy of Mind, 13 June 2002 http://www.swif.uniba.it/lei/mind/kim1.htm.
Quoted from

Précis of
Jaegwon Kim. Mind in a Physical World: An Essay on the Mind-Body problem and Mental Causation.
"Representation and Mind Series". Cambridge (Mass.): A Bradford Book, The MIT Press, 1998.

Jaegwon Kim
Department of Philosophy
Brown University

(To be published as part of a book symposium on "Mind in a Physical World" in Philosophy and Phenomenological Research, with essays by Frank Jackson, Marcelo Sabates, Barry Loewer, and Pierre Jacob and responses by Jaegwon Kim)

The main claim of Mind in a Physical World is that any physicalist who wants to save the causal efficacy of mentality must be prepared to embrace physical reductionism. I have argued this point before in various places – in scattered journal articles and book chapters. In this small book I try to give a more focused and unified argument for this conclusion by sifting through the many considerations and reconsiderations that have over the past dozen years or so pushed me, slowly but inexorably, toward a more strict physicalist position, away from a relaxed supervenience-based nonreductionist view. Along the way, I discuss several other related issues, such as supervenience physicalism, the idea of realization, the doctrine of emergence, and models of reduction. I also comment on various nonreductionist solutions to the mental causation problem.

For the physicalist, the mind-body problem is the problem of finding a place for the mind in a world that is fundamentally physical. What does "fundamentally physical" mean? I think any physicalist will accept at least the following two claims. First, the world contains nothing but bits of matter and aggregates of bits of matter. There are no Cartesian souls, or Hegelian spirits, or neo-vitalist entelechies – as the emergentist C. Lloyd Morgan put it, no "alien influx" into the natural order. This ontological thesis is sometimes called "ontological physicalism" (perhaps, "substance physicalism" is a better name). Second, we have the supervenience thesis: physical facts determine all the facts, and the physical properties of a thing determine all its properties (here, "properties" can be read narrowly – to mean, intrinsic properties – or broadly – to include relational/extrinsic properties as well). At this point, then, the world looks like this: all the things that exist are physical things – either basic bits of matter or wholly made up of bits of matter. These physical things have properties. What properties? First, there are basic physical properties, like mass, size, shape, electric charge, and so on – properties and magnitudes in terms of which laws of physics are formulated. But some properties of complex physical systems, like biological organisms, seem prima facie nonphysical – at least, they are not among the properties investigated in physics or the physical sciences. Prominent among them are mental properties – beliefs, desires, sensations, emotions, and the rest. The supervenience thesis says that such properties are fixed by the physical properties of the systems that have them; once the physical properties of a system are fixed, that fixes all of its properties. And yet property dualists, or nonreductive physicalists, maintain that although certain nonphysical properties are determined by physical properties, they are irreducible to, and remain distinct from, physical properties. Although thoughts and pains are determined by the biology, and ultimately physicochemistry, of an organism, they are not biological/physical properties themselves, and there exists a special autonomous science of psychology, or cognitive science, that is responsible for investigating them.

A view like this has been the dominant position concerning the status of mentality and other "higher-level" properties investigated in the special sciences. These properties, though supervenient upon and "realized" by fundamental physical properties, are irreducible and autonomous, and they enter into physically irreducible causal laws and causal explanations. This gives us an attractive view of the status of the special sciences that many seem to find irresistible: Physics is not all there is to science; the special sciences do not merely redescribe the phenomena described, more precisely and in richer detail, by physics, and the explanations they provide are not mere stopgap measures that only cloak our ignorance of the physical processes involved.

A further thesis that I believe any physicalist should, and would, accept is the causal closure of the physical domain. This can be stated in various ways that are not strictly equivalent to each other. One way to put it would be this: If a physical event has a cause that occurs at t, it has a physical cause occurring at t. A stronger version would go like this: No physical event has a cause outside the physical domain.

I think it would be quite evident to everyone that given these physicalist doctrines the prospect of mentality turning out to be both (i) causally efficacious in the physical world and yet (ii) not being part of the physical domain looks pretty grim. In the book, I advance various considerations to convince the reader why it really is grim – in fact, why it is completely hopeless. My main argument is what I call "the supervenience argument", sometimes also called "the exclusion argument" in the literature. The gist of this argument is that when we consider the claim that a certain mental event, M, is a cause of another event, R, we see that M’s physical supervenience base, P, has all the credentials to serve as a cause of R, thus threatening to preempt M’s claim to be a cause of R. The only way out for the nonreductivist would seem to be to recognize both M and P each as a full cause of R, making R causally overdetermined. I do not believe this is a plausible move for the physicalist to make (for one thing, it may well violate physical causal closure), but some nonreductivists seem willing to embrace it. In contrast, the reductionist has a simple answer: If M is to retain its causal status, it must be reducible to P – at least, the given instantiation of M must be reductively identifiable with the instantiation, on that occasion, of its supervenience or realization base.

This calls for an explanation of how we should understand reduction. I offer arguments against the standard Nagelian model of reduction according to which the reduction of a higher-level property M to a lower-level property P requires an empirical "bridge law" of the form "M iff P". Two of the most influential antireductionist arguments -- the Putnam-Fodor multiple realization argument and Davidson’s anomalism argument -- have focused on showing that the bridge-law requirement cannot be met. I argue that Nagelian bridge laws are neither necessary nor sufficient for proper reductions. In place of the Nagel model, I propose a model of functional reduction (this is not something wholly new; the basic ideas go back to David Lewis and David Armstrong in the ‘60s and are present in the works of philosophers like Joseph Levine and David Chalmers). A functional reduction of a property works like this: (step i) the property must first be construed as a second-order functional property through a functional definition, a definition in terms of its "causal role"; (step ii) we must then find its "realizers", that is, first-order properties that fit the causal role specified by the functional definition; and (step iii) we must have an explanatory theory that explains how the realizers of the property perform the specified causal tasks. (i) is a conceptual step, though it is likely to reflect numerous empirical/theoretical constraints and desiderata; (ii) and (iii) are in the province of empirical scientific research.

I argue that functional reductions are indeed reductions: they deliver ontological simplification and yield explanations of reduced phenomena at the level of their reduction base. My concluding claim is that a functional reduction of mentality could vindicate mental causation on the physicalist terms.

Suppose that mental property M has been functionally reduced, with each instance of M being reductively identified with an instance of one of M’s physical realizers. This gives us a reduction of M-instances. But what happens with property M? Does M itself get reduced? Or does M stay around unreduced, a property "over and above" physical/biological properties? All we need to worry about in the matter of M’s reduction, I believe, is the reduction of all M-instances; there are no significant philosophical issues beyond that. However, if one is concerned with the ontological fate of M, there are two possibilities. The first option is this: we deny that there are such things as "second-order" properties (note: functional properties are standardly construed as a species of second-order properties); there are only second-order predicates, expressions, and concepts. By forming second-order expressions we do not bring into existence new properties; we only introduce new ways of talking about the properties already in hand. This is an irrealist, a sort of eliminativist, option. Second, there is a conservative (preservative or retentive, as some would put it) option: Identify M with the disjunction of its realizers. The disjunction will have indefinitely many disjuncts, but I don’t see that as a problem. We of course need an ontology that allows such disjunctions, but I don’t see a deep problem here either. On this approach, M will be retained as a disjunctive property – causally and nomologically heterogeneous and unprojectible, but a property nonetheless.

But are mental properties physically reducible? I have argued that if they are to be causally efficacious, whether with respect to physical properties or other mental properties, they must be reducible to physical properties. But to "solve" the problem of mental causation, another step must be taken: we need to show that mental properties are in fact physically reducible. The position I favor here is similar to the position recently defended by several philosophers: (i) cognitive/intentional mental properties, including belief, desire, perception, and the like are physically reducible (via functional reduction); however, (ii) sensory qualities of conscious experience ("qualia") are not so reducible. The book offers no substantially new argument for these two claims.

To summarize, then, the problem of mental causation is solvable for cognitive/intentional mental properties. But it is not solvable for the qualitative or phenomenal characters of conscious experience. We are therefore left without an explanation of how qualia can be causally efficacious; perhaps, we must learn to live with qualia epiphenomenalism. But the positive outcome for cognitive/intentional properties means that we have saved our status as cognizers and agents. Isn’t the possible loss of agency and cognition what has worried us most about the problem of mental causation? In the end, then, our cup isn’t quite full, but we see that it is more than half full.

© 2002 Jaegwon Kim