Abstract
The literature on physicalism often fails to elucidate, I think, what the word physical in physicalism precisely means. Philosophers speak at times of an ideal set of fundamental physical facts, or they stipulate that physical means non-mental, such that all fundamental physical facts are fundamental facts pertaining to the non-mental. In this article, I will probe physicalism in the very much tangible framework of quantum mechanics. Although this theory, unlike “ideal physics” or some “final theory of non-mentality”, is an incomplete theory of the world, I believe this analysis will be of value, if for nothing else, at least for bringing some taste of physical reality, as it were, back to the debate. First, I will introduce a broad characterization of the physicalist credo. In Sect. 2, I will provide a rather quick review of quantum mechanics and some of its current interpretations. In Sect. 3, the notion of quantum non-separability will be analyzed, which will facilitate a discussion of the wave function ontology in Sect. 4. In Sects. 5 and 6, I will explore competing views on the implications of this ontology. In Sect. 7, I will argue that the prior results, based on a thoroughly realist interpretation of quantum mechanics, support only a weak version of non-reductive physicalism.
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Notes
Cf., for example, Nagel (1961), Strawson (1963), Nagel (1965, 1974), Peacocke (1979), McGinn (1982), Fodor (1974, 1987), Kim (1993, 1997), Loewer (1995, 2001), Jackson (1996, 1998), Armstrong (1997), Sturgeon (1998), Papineau (1993, 2001), Wilson (2005, 2006), Melnyk (1997, 2003, 2006), Witmer (1997, 2006).
Cf. Davidson’s “Mental Events”, in Davidson (1980).
Wilson, for example, affirms: “the physical entities are the relatively fundamental entities treated by current or future (in the limit of inquiry, ideal) physics”, and she adds: “It is also required…that the physical entities not be fundamentally mental” (2009, p. 2). I imagine the intuition behind such complacent view, which I deem mistaken, is that the ultimate fate of the physicalist doctrine does not really depend on what the word “physical” exactly means. For further discussion of this problem, see Chomsky (1972, 1980, 1993), Hempel (1969, 1980), Lewis (1983), Stroud (1987), Crane and Mellor (1990), Poland (1994), Melnyk (1997, 2003), Montero (1999), Crook and Gillett (2001), Wilson (2006), Stoljar (2006).
This makes physicalism trivially false: there are objects, properties, or events that are not, even if physical, quantum-mechanical. It is in view of this result that philosophers insist on the completeness of “ideal physics”. Jackson, for example, stipulates that the physical must include “everything in a completed physics, chemistry, and neurophysiology, and all there is to know about the causal and relational facts consequent upon all this” (1991, p. 291). As Loewer puts this, what philosophers “have on their minds when they speak of fundamental physical properties is that they are the properties expressed by simple predicates of the true comprehensive fundamental physical theory” (1996, p. 103).
Lewis (2007) has recently defended this “orthodox” account against a “recurring heresy” according to which there is only one interpretation that solves the problem, i.e., the Many-Worlds interpretation.
Mermin’s Ithaca Interpretation of Quantum Mechanics, which I have criticized in (2009), is as well a corollary of Everett’s tradition—Mermin (1998a, b). An alternative line of research inspired by Everett’s ideas involves the derivation of all predictions associated with principle (5) from the consideration of decision-theoretic principles—cf., for example, Deutsch (1999),Wallace (2003, 2007), Greaves (2007).
There exist dissenting voices, however, for which the universal wave function falls short of being real in this metaphysical sense. Durr et al. (1997), for example, claim: “the wave function of the universe is not an element of physical reality. We propose that the wave function belongs to an altogether different category of existence than that of substantive physical entities, and that its existence is nomological rather than material. We propose, in other words, that the wave function is a component of physical law rather than of the reality described by the law” (p. 10). And Bohm himself was skeptical about this. He argued (1957, p. 117): “While our theory can be extended formally in a logically consistent way by introducing the concept of a wave in a 3 N-dimensional space, it is evident that this procedure is not really acceptable in a physical theory”, although he failed to elaborate on why such procedure would be evidently unacceptable.
Cf. Bell (1987, p. 128), where he says: “No one can understand this [Bohm’s] theory until he is willing to think of ψ as a real objective field rather than just a “probability amplitude”. Even though it propagates not in 3-space but in 3 N-space”.
It is worth noting here that, unlike other analyses of physicalism from a perspective that pays heed to quantum theory, such as, to mention a well-known example, Chalmers (1996), Chap. 10, the analysis that comes next in this paper does not depend on any idiosyncratic feature of any specific interpretation of this theory. More precisely, although the details of how a system’s physical properties relate to the quantum-mechanical formalism are still in this analysis a function of the specific interpretation at hand—for a consistent study of how distinct interpretations uphold the attributes of definiteness and determinism differently, see Dickson (1998, Chaps. 2–5), its results are not contingent on any one interpretation being right. And thus our emphasis on working at once under alternative approaches to the theory.
Lewis (2004), however, has argued that it is quite possible to be serious about quantum mechanics and the wave function ontology (i.e., to endorse a non-instrumentalist reading of quantum mechanics in which the universal wave function is real) without necessarily being serious about 3 N-configuration space (i.e., without believing that 3 N-configuration space is a real space, with real topological structure, and not only a mathematical contraption). It is far from evident, however, whether Lewis—whose central contention is that, in some sense, “configuration space in fact has three dimensions rather than 3 N” (ibid., p. 725)—has succeeded in showing that there exists such a possibility. Cf. Monton (2006, pp. 786–789).
Albert, for example, strongly defends this standpoint. He affirms: “… the space in which any realistic understanding of quantum mechanics is necessarily going to depict the history of the world as playing itself out… is configuration-space” (p. 277).
In GRW, the universal wave function right after the collapse has tails: it remains, strictly speaking, spread all over the place.
If these physical facts taking place in position space could be counted as the fundamental physical facts, and if (as presupposed here) everything supervened on these physical facts, then at least some version of non-reductive physicalism would be trivially guaranteed. The problem is, of course, that particle facts in position space are not the fundamental physical facts in the wave function ontology. And physicalism, unlike restricted versions of the old Mind-Body problem, is a thesis concerned with the exact correspondence between all rich and dissimilar aspects of reality and the fundamental realm of physical reality.
This will not be necessarily an exegesis of Monton’s analysis but rather a reconstruction of what, in my view, Monton’s challenge amounts to.
Discussing this possibility, which seems indeed correct, would bring up thorny questions about haecceitism, or the thesis that worlds which only differ from each other by way of a simple permutation in the position of otherwise identical particles are nonetheless different, that we should rather avoid here.
A similar objection against Monton’s argument has been raised by Wallace and Timpson (2009). I want to thank one of the referees for pointing this out to me.
Albert, however, seems more amenable to a “mixed ontology” in (2000). He says: “… a list of what particles exist, and what sorts of particles they are, and of what their positions are at all times, is a list of absolutely everything there is to say [in a Newtonian framework] about the physical history of the world”, and then he adds that “this is certainly not to deny that there are such things in the world as extended objects; the idea is just that all the facts about objects like that (facts about where the tables and chairs are, and about who punched whom, and about who said what, and so on) are determined, in principle, by the facts about the particles of which those objects are composed” (p. 1). I think it is reasonable to wonder here why a mixed ontology should be abandoned when we move, at the fundamental level, from the Newtonian ontology of point particles to the wave function ontology. But Albert does not provide any clue, either in (1996) or (2000), of why that should be the case.
Wallace and Timpson, along similar lines, affirm: “While the wave-function realist will deny that three-dimensional objects and spatial structures find a place in the fundamental ontology, this is not to say that the three dimensional objects surrounding us, with which we constantly interact, and which we perceive, think and talk about, do not exist; that there are not truths about them; it is just to maintain that they are emergent objects, rather than fundamental ones. But an emergent object is no less real for being emergent” (2009, p. 8).
He says (2006, p. 781): “I have nothing definitive to say about this ontology, other than that it strikes me as strange. We have two disconnected spaces, with presumably no causal connection between the particles in the one space and the field in the other space, and yet the stuff in the two spaces is evolving in tandem. Presumably there is a nomic connection between the stuff in the two spaces, which supports counterfactuals of the following form: if the stuff in the one space had evolved differently, the stuff in the other space would have evolved differently. But having the nomic connection without a causal connection makes it all the more mysterious how these spaces are associated with each other”. I find Monton’s demand for a causal connection along with a nomic connection rather perplexing. Under a Lewisian conception of laws, the correlation between particle evolution and wave function evolution is indeed law-like. But such counterfactual dependence between their respective evolutions determines as well a causal connection. If, on the other hand, this demand for a causal connection is understood in terms of some narrative that necessarily arises from the fundamental realm, in line with a strict necessitarian version of the deductive-nomological model of causation, then this demand is strangely severe and hardly the norm in science.
There exist different versions of the anthropic principle in physics and cosmology, which serve different purposes. Broadly speaking, it establishes that “scientists who study nature must live in a part of the landscape [i.e., the set of possible configurations] where physical parameters take values suitable for the appearance of life and its evolution into scientists” (Weinberg, 2007, p. 31). When confronted with the strong distaste that some physicists feel for this principle, Weinberg observes (p. 39) that “Theories based on anthropic calculations certainly represent a retreat from what we had hoped for: the calculation of all fundamental parameters from first principles. It is too soon to give up on this hope, but without loving it we may just have to resign ourselves to a retreat, just like Newton had to give up Kepler’s hope of a calculation of the relative sizes of planetary orbits from first principles”.
Under a Lewisian conception of laws, as just noted, the instantiation of such correlation may well be counted as a physical law itself. Thanks to one of the referees for the point.
A more or less similar formulation can be devised for single properties and events.
Some philosophers contend at present that this supervenience is likely global rather than local, either in a strong or weak version. Since strong supervenience entails (cf. Kim 1993) global supervenience, we will postulate here a strong version of non-reductive physicalism and stick to the strong supervenience of macroscopic and intentional facts on particle facts.
It seems clear that a rejection of strong non-reductive physicalism entails the rejection of reductive physicalism. If x does not strongly supervene on y, then x cannot possibly be the same as y. For a different take on this, however, see Davidson (1980) and his defense of his so-called anomalous monism. For Davidson, although this is still controversial, the identity thesis and the supervenience thesis, both of which he supports, are not logically connected, which would presumably make it possible for the supervenience thesis to be false while the identity thesis is true.
For Sturgeon, there are two critical assumptions behind the over-determination argument for physicalism: the completeness of physics (COP), and the impact that the mental has on the physical (IMP). The argument maintains that, in order to avoid the over-determination of physical events, we must recognize that the impact that the mental has on the physical is a physical event itself, in line with the—presumed—completeness of physics. But Sturgeon claims: “if “physical” means quantum mechanical, (COP) seems supported by science; but (IMP) seems supported neither by science nor by everyday experience. If “physical” means broadly physical, (IMP) seems supported by science and everyday experience; but (COP) seems supported by neither”, which entails that “The plausibility of (COP) and (IMP) trade on distinct readings of “physical”. The O-Argument [i.e., the over-determination argument] is unsatisfactory as it stands” (1998, p. 416). And Sturgeon takes this to entail that physicalism is not “the only scientifically respectable worldview” (p. 411), or “the only respectable game in town” (ibid., p. 430).
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Birman, F. Quantum Mechanics and the Plight of Physicalism. J Gen Philos Sci 40, 207–225 (2009). https://doi.org/10.1007/s10838-009-9104-2
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DOI: https://doi.org/10.1007/s10838-009-9104-2