In philosophy, physicalism is the ontological thesis that "everything is physical", or that there is "nothing over and above" the physical. Physicalism, therefore, is a form of ontological monism—a "one stuff" view of the nature of reality as opposed to a "two-stuff" (dualism) or "many-stuff" (pluralism) view. Two questions immediately arise. First, what is meant by "the physical"? Second, what is it for non-physical properties to be "nothing over and above" the physical?
Physicalism is closely related to materialism.
Characterizing "the physical"
In answer to the first question (What is meant by "the physical"?), physicalists have traditionally opted for a "theory-based" characterization of the physical either in terms of current physics, or a future (ideal) physics. These two theory-based conceptions of the physical represent both horns of Hempel's dilemma (named after the late philosopher of science and logical empiricist Carl Gustav Hempel): a well-known argument against theory-based understandings of the physical. Very roughly, Hempel's dilemma is that if we define the physical by reference to current physics, then physicalism is very likely to be false, as it is very likely (by pessimistic meta-induction) that much of current physics is false. But if we instead define the physical in terms of a future (ideal) or completed physics, then physicalism is hopelessly vague or indeterminate. While the force of Hempel's dilemma against theory-based conceptions of the physical remains contested, alternative "non-theory-based" conceptions of the physical have also been proposed. Frank Jackson (1998) for example, has argued in favour of an "object-based" conception of the physical whereby (roughly speaking) an object or property is physical if and only if it is either a paradigmatic example of the physical, such as a rock or a tree, or it is required for a complete account of such entities or properties. An objection to this proposal, which Jackson himself noted in 1998, is that if it turns out that panpsychism or panprotopsychism is true, then the aforementioned understanding of the physical gives the incorrect (for some anyway) result that physicalism is, nevertheless, also true since such properties will figure in a complete account of paradigmatic examples of the physical. Finally, David Papineau and Barbara Montero have advanced and subsequently defended a "via negativa" characterization of the physical. The gist of the via negativa strategy is to understand the physical in terms of what it is not: the mental. In other words, the via negativa strategy understands the physical as "the non-mental". An objection to the via negativa conception of the physical is that (like the object-based conception) it doesn't have the resources to distinguish neutral monism (or panprotopsychism) from physicalism.
The physicalist slogan is: "There is nothing over and above the physical." The previous section addresses the issue of how "the physical" is to be understood. This section is concerned with how physicalists unpack their claim that there is "nothing over and above" the physical. Most physicalists understand "nothing over and above" in terms of "metaphysical supervenience". Supervenience is generally thought of as a "determination" relation, which entails that if "A" properties supervene upon "B" properties, then there cannot be a change or difference in A properties without a change or difference in B properties. "Metaphysical" supervenience entails that if A properties metaphysically supervene upon B properties, then at every possible world in which A properties and B properties are instantiated, there cannot be a difference in A properties without a difference in B properties. If physicalism is understood in terms of metaphysical supervenience, then the thesis at issue is that all non-physical properties in the actual world metaphysically supervene upon the physical. So understood, physicalism does not entail that all properties in the actual world are type identical to physical properties. It is, therefore, compatible with multiple realizability. If all non-physical properties in the actual world metaphysically supervene upon the physical, then there cannot be a world that is just like ours in all physical respects, but which differs from ours in some non-physical respect—on pain of there being a difference in non-physical properties without a difference in physical properties, thus contradicting the original supervenience claim. This suggests a natural supervenience-based formulation of physicalism:
Applied to the actual world (our world) 1) is the claim that physicalism is true at the actual world if and only if at every possible world in which the physical properties and laws of the actual world are instantiated, the non-physical properties of the actual world are instantiated as well. To borrow a metaphor from Saul Kripke (1972), the truth of physicalism at the actual world entails that once God has instantiated or "fixed" the physical properties and laws of our world, then God's work is done; the rest comes "automatically". Unfortunately, 1) fails to capture even a necessary condition for physicalism to be true at a world w. To see this, imagine a world in which there are only physical properties—if physicalism is true at any world it is true at this one. But surely there are physical duplicates of such a world that are not also duplicates simpliciter of it: worlds that have the same physical properties as our imagined one, but with some additional property or properties. To handle this problem, 1) can modified to include a "that's-all" or "totality" clause or be restricted to "positive" properties. Adopting the former suggestion here, we can then reformulate 1) as follows:
2) Physicalism is true at a possible world w if and only if any world that is a minimal physical duplicate of w is a duplicate of w simpliciter.
Applied in the same way, 2) is the claim that physicalism is true at a possible world w if and only if any world that is a physical duplicate of w and that's-all, or "stop-right-there", is duplicate of w without qualification. This allows a world in which there are only physical properties to be counted as one at which physicalism is true, since worlds in which there is some extra stuff are not "minimal" physical duplicates of such a world, nor are they minimal physical duplicates of worlds that contain some non-physical properties that are metaphysically necessitated by the physical. But while 2) overcomes the problem of worlds at which there is some extra stuff (sometimes referred to as the "epiphenomenal ectoplasm problem".) it faces a different challenge: the so-called "blockers problem". Imagine a world where the relation between the physical and non-physical properties at this world (call the world w1) is slightly weaker than metaphysical necessitation, such that a certain kind of non-physical intervener—"a blocker"—could, were it to exist at w1, prevent the non-physical properties in w1 from being instantiated by the instantiation of the physical properties at w1. Since 2) rules out worlds which are physical duplicates of w1 that also contain non-physical interveners by virtue of the minimality, or that's-all clause, 2) gives the (allegedly) incorrect result that physicalism is true at w1. One response to this problem is to abandon 2) in favour of the possibility mentioned earlier of restricting supervenience-based formulations of physicalism to what David Chalmers (1996) calls "positive properties", where a positive property is one that "...if instantiated in a world W, is also instantiated by the corresponding individual in all worlds that contain W as a proper part." Following this suggestion, we can then formulate physicalism as follows:
3) Physicalism is true at a possible world w if an only if any world that is physical duplicate of w is a positive duplicate of w.
On the face of it, 3) seems able to handle both the epiphenomenal ectoplasm problem and the blockers problem. With regard to the former, 3) gives the correct result that a purely physical world is one at which physicalism is true, since worlds in which there is some extra stuff are positive duplicates of a purely physical world. With regard to the latter, 3) appears to have the consequence that worlds in which there are blockers are worlds where positive non-physical properties of w1 will be absent, hence w1 will not be counted as a world at which physicalim is true. Daniel Stoljar (2010) objects to this response to blockers problem on the basis that since the non-physical properties of w1 aren't instantiated at a world in which there is a blocker, they are not positive properties in Chalmers' (1996) sense, and so 3) will count w1 as a world at which physicalism is true after all.
A further problem for supervenience-based formulations of physicalism is the so-called "necessary beings problem". Imagine a non-physical being that exists in all possible worlds (no doubt some theists consider God such a being). If there is such a being, then it is natural to believe that physicalism is false. But supervenience-based formulations of physicalism are compatible with the existence of a non-physical necessary being, therefore supervenience-based formulations of physicalism at best state a necessary not a sufficient condition for the truth of physicalism.
A priori versus a posteriori physicalism
Physicalists hold that physicalism is true. A natural question for physicalists, then, is whether the truth of physicalism is knowable a priori (i.e., with justification independent of experience) or a posteriori (i.e., with justification dependent upon experience). So-called "a priori physicalists" hold that from knowledge of the conjunction of all physical truths, a totality or that's-all truth, and some primitive indexical truths, the truth of physicalism is knowable a priori. Let "P" stand for the conjunction of all physical truths and laws, "T" for a that's-all truth, "I" for the indexical "centering" truths "I am A" and "now is B", and "N" for any non-physical truth at the actual world. We can then, using the material conditional "→", represent a priori physicalism as the thesis that PTI → N is knowable a priori. An important wrinkle here is that the concepts in N must be possessed non-deferentially in order for PTI → N to be knowable a priori. The suggestion, then, is that possession of the concepts in the consequent, plus the empirical information in the antecedent is sufficient for the consequent to be knowable a priori. One profound challenge to a priori physicalism and to physicalism in general is the "conceivability argument", or zombie argument. At a rough approximation, the conceivability argument runs as follows:
P1) PTI and not Q (where "Q" stands for the conjunction of all truths about consciousness, or some arbitrary truth about someone being "phenomenally" conscious [i.e., there is "something it is like" to be a person x] ) is conceivable (i.e., it is not knowable a priori that PTI and not Q is false).
P2) If PTI and not Q is conceivable, then PTI and not Q is metaphysically possible.
P3) If PTI and not Q is metaphysically possible then physicalism is false.
C) Physicalism is false.
Since a priori physicalists hold that PTI → N is a priori, they are committed to denying P1) of the conceivability argument. The a priori physicalist, then, must argue that PTI and not Q, on ideal rational reflection, is incoherent or contradictory.
A posteriori physicalists, on the other hand, generally accept P1) but deny P2)--the move from "conceivability to metaphysical possibility". Some a posteriori physicalists think that unlike the possession of most, if not all other empirical concepts, possession of the concept of consciousness entails that the presence of PTI and the absence of consciousness will (always) be conceivable—even though, according to them, it is knowable a posteriori that PTI and not Q is not metaphysically possible. These a posteriori physicalists endorse some version of what Daniel Stoljar (2005) has called "the phenomenal concept strategy". Roughly speaking, the phenomenal concept strategy is a label for those a posteriori physicalists who attempt to show that it is only the concept of consciousness—not the property—that is in some way "special" or sui generis. Other a posteriori physicalists eschew the phenomenal concept strategy, and argue that even ordinary macroscopic truths such as "water covers 60% of the earth's surface" are not knowable a priori from PTI and a non-deferential grasp of the concepts "water" and "earth" et cetera. If this is correct, then we should (arguably) conclude that conceivability does not entail metaphysical possibility, and P2) of the conceivability argument against physicalism is false.
One can be both a physicalist and a panpsychist – or even a monistic idealist. According to Galen Strawson, physicalists acknowledge the world is exhaustively described by the equations of physics. There is no "element of reality", as Einstein puts it, that is not captured in the formalism of theoretical physics—the quantum-field theoretic equations and their solutions. However, physics gives us no insight into the intrinsic nature of the stuff of the world—what "breathes fire into the equations and makes there a world for us to describe" as renowned materialist Stephen Hawking poetically laments. Key terms in theoretical physics like "field" are defined purely mathematically.
Is the intrinsic nature of the physical, the "fire" in the equations, a wholly metaphysical question? Kant famously claimed that we would never understand the noumenal essence of the world, merely phenomena as structured by the mind. Strawson, drawing upon arguments made by Oxford philosopher Michael Lockwood but anticipated by Russell, disagrees with Kant. Strawson suggests there is one part of the natural world that we do know as it is in itself, and not at one remove, so to speak—and its intrinsic nature is disclosed by subjective properties of one's own conscious mind. Thus it transpires that the "fire" in the equations is utterly different from what one's naive materialist intuitions would suppose.
- See Smart, 1959
- See e.g., Smart, 1978; Lewis, 1994.
- See e.g., Poland, 1994; Chalmers, 1996; Wilson, 2006.
- Andrew Melnyk should apparently be credited with having introduced this name for Hempel's argument. See Melnyk, 1997, p.624
- see Vincente, 2011
- See Hempel, 1969, pp.180-183; Hempel, 1980, pp.194-195.
- For a recent defence of the first horn see Melnyk, 1997. For a defence of the second, see Wilson, 2006.
- See Jackson, 1998, p.7; Lycan, 2003.
- See Papineau, 2002
- See Montero, 1999
- See Papineau and Montero, 2005
- See e.g., Judisch, 2008
- See Bennett and McLaughlin, 2011
- See Bennett and McLaughlin, 2011, section 3.1
- See Putnam, 1967
- See Jackson, 1998
- see Jackson, 1998
- see Chalmers, 1996
- See Jackson, 1998
- Where "metaphysical necessitation" here simply means that if "B" metaphysically necessitates "A" then any world in which B is instantiated is a world in which A is instantiated--a consequence of the metaphysical supervenience of A upon B. See Kripke, 1972.
- See e.g., Stoljar, 2009, section 4.3.
- See Hawthorne, 2002.
- Chalmers, 1996, p.40.
- See Chalmers, 1996; Stoljar, 2009, section 4.3.
- see Hawthorne, 2002, p.107
- See Stoljar, 2010, p.138
- See Jackson, 1998
- See Jackson, 1998
- See Chalmers and Jackson, 2001
- See Chalmers and Jackson, 2001
- See Chalmers, 2009.
- See Nagel, 1974
- See Chalmers, 2009
- For a survey of the different arguments for this conclusion (as well as responses to each), see Chalmers, 2009.
- See Stoljar, 2005
- cf. Stoljar, 2005
- e.g., Tye, 2009
- For critical discussion, see Chalmers, 2009.
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Daniel Stoljar's SEP entry on Physicalism: http://plato.stanford.edu/entries/physicalism/