Abstract
Though there is yet no consensus on the right way to understand ‘physicalism’, most philosophers agree that, regardless of whatever else is required, physicalism cannot be true if there exists fundamental mentality. I will follow Jessica Wilson (Philosophical Studies 131:61–99, 2006) in calling this the 'No Fundamental Mentality' (NFM) constraint on physicalism. Unfortunately for those who wish to constrain physicalism in this way, NFM admits of a counterexample: an artificially intelligent quantum computer which employs quantum properties as part of its cognitive operations. If one of these quantum properties serves a proper functional role in the artificial intelligence, then that property counts as a mental under the physicalism-friendly theory of mentality called “realizer functionalism”, which says that a lower-order property is mental if it satisfies an appropriate higher-order functional description. Further, if this quantum property is both fundamental and mental, then NFM must rule that it is not physical. Yet the existence of such an artificially intelligent quantum computer, which possesses mental properties solely in virtue of the functional roles those properties play, is surely consistent with the truth of physicalism. This ought to motivate NFM proponents to reformulate their view.
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Notes
Notable exceptions are Galen Strawson (2003), who thinks that physicalism is identical to panpsychism; Noam Chomsky (1995), who thinks that physicalism is trivially true; Daniel Stoljar (2001) who thinks that exemplars of physical objects might turn out to have fundamental mentality, and that these exemplary physical objects define what is physical; and Janice Dowell (2006), who thinks that ideal physical theory is consistent with the existence of fundamental mentality.
I will use “fundamentally mental” and “fundamental mentality” interchangeably. Further, I will assume that if something is both fundamental and mental, then it is fundamentally mental (or, equivalently, that it is an instance of fundamental mentality). However, when Wilson (2006, p. 68) says that fundamentally mental entities are “such as to individually possess or bestow mentality”, she perhaps has stricter requirements for determining whether something counts as fundamentally mental. Regardless, the problem case I will discuss is an instance of a property individually bestowing mentality, so it ought to satisfy Wilson’s (perhaps) stricter requirements.
Dorsey (2011) and Zhong (2016) raise a similar issue for NFM, both claiming that it is conceivable that fundamental entities which seem physically acceptable could be mental entities. The argument presented in this paper can be seen as an attempt at fleshing out their claim, to show how such physically acceptable fundamental mentality might be instantiated in the actual world.
Note the requirement that realizer functionalism must relativize mental properties to populations, such that a mental property M is identical with whatever realizer property P in population N plays the M role in all members of N (thanks to an anonymous referee for pushing me on this). Relativization to a population type must be done to avoid the problem that multiple first-order properties might satisfy a functional description, which would have failure of identification as a consequence if a mental property is specified as the realizer of some functional description. However, there are multiple plausible ways to go about doing the requisite relativization—specifically, one can relativize to a species, or relativize to a smaller population type (perhaps even to a population type which contingently has a sole member). For the present argument, it is not important what sort of population mental properties are relativized to.
Note that nothing in my argument requires multiple realizability of mental properties. NFM rules that physicalism is false if there exists any fundamental mentality, regardless of whether that mentality is multiply realizable or not. Under realizer functionalism, mental properties are identified with the first-order realizers of functional role descriptions, and so are not multiply realizable, since this view is ultimately a sort of identity theory. For example, suppose that human pain = neural property N, martian pain = alien goo property M, and computer pain = silicon property S. Since N ≠ M ≠ S, it follows that the realizers in these cases are non-identical, so human pain cannot be possessed by computers or Martians, even though the functional description of human, Martian and computer pain might be identical. It may seem that realizer functionalism requires or allows multiple realizability because this view has us apply the same functionalist criteria across the board in order to determine mentality ascription, e.g. whether property P is pain or a pain-like mental property depends on whether P is caused by e.g. body damage and causes pain-appropriate-thoughts and avoidance behavior. It does not follow from the view that satisfaction of an appropriate functional description suffices for mentality ascription that all satisfiers are the same mental property—in fact, this is exactly what realizer functionalism denies. That said, it is nonetheless the case that mental properties fall under a common functionally-defined genus under realizer functionalism, even though they are non-identical mental properties.
Technically, qubits can be represented as positions on a three-dimensional Bloch sphere (which requires the use of imaginary numbers to accurately represent), but as I understand it the third dimension is largely irrelevant for quantum computational purposes.
Note that this view is almost certainly false: nearly all of cognitive science is predicated on the assumption that some (or, more likely, all) mentality-relevant functions are Turing-computable. Progress in cognitive science has largely been based on accurately modeling cognitive processes as Turing-computable functions, e.g. Marr’s (1982) work on vision.
Again, this does not require that quantum computers are hypercomputers, nor that they are able to compute functions in logarithmic time that are only computable by classical computers in exponential time. All that is required is that quantum computers are measurably more powerful than classical computers, which has now been well-demonstrated experimentally (see Arute et al., 2019).
I admit that it is difficult to understand what ‘strong emergence’ means. Wilson (2011) defines ‘strong emergence’ as occurring when a high-level property has causal powers which are not a proper subset of the causal powers of its subvenience base, but this excludes epiphenomenal properties from possibly being strongly emergent, which seems wrong. I prefer to understand ‘strong emergence’ relative to different modal constraints: if the existence of some high-level property is nomologically necessary but not metaphysically necessary, then that property is strongly emergent. Though this articulation faces problems if all properties possess their nomological profiles with metaphysical necessity, as under necessitarianism (see Bird, 2005). Regardless of exactly how to cash out ‘strong emergence’, I take it that it is plausible that (i) strongly emergent properties are fundamental in some important sense, and (ii) entangled properties are strongly emergent.
I would not say the same thing about not throwing out NFM because of a strange case concerning quantum computation, since NFM explicitly concerns fundamental mentality, whereas issues with fundamental mentality are obviously peripheral to realizer functionalism.
There is a substantive question lurking here about whether our mental concepts are rigid designators—such that they refer to the same referents in all worlds in which those concepts refer—or non-rigid designators—such that the same concept refers to different referents in different possible worlds. Lewis (1980) develops a version of realizer functionalism under which mental concepts are non-rigid, but I take it that nothing bars realizer functionalists from deeming mental concepts to be rigid. The issues with rigidity and realizer functionalism are difficult. I would like to avoid taking a stance on the rigidity/non-rigidity of mental concepts under realizer functionalism, since I believe that my argument is viable either way. However, note that a host of complexities and potential problems arise if mental terms are taken to be non-rigid, and that it is perhaps more natural for realizer functionalism to deem mental terms as rigid rather than non-rigid designators.
Perhaps Alter could reject my argument in Sect. 5, and hold that even if categoricalism is true, the case I describe—of quantum mechanical realizer functionalist mentality—is one in which fundamental mental properties partially metaphysically depend on function-appropriate structural/dynamical relations. However, even if it is true that the realizer mental properties under realizer functionalism partially metaphysically depend on structural/dynamical properties, this does not save Alter’s view. This is because many dualists also claim that non-physical mental properties partially depend on realization of appropriate functional properties—for instance Chalmers’ (1996) psychophysical law dualism. Perhaps a view such as psychophysical law dualism can be distinguished from realizer functionalism on the grounds that psychophysical law dualism requires mentality-specific psychophysical laws. However, such a project would go beyond characterizing the non-physical as merely non-structural/non-dynamical mentality, and so Alter’s NNM principle would still be insufficient.
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Brown, C.D. Quantum computation and the untenability of a “No fundamental mentality” constraint on physicalism. Synthese 201, 10 (2023). https://doi.org/10.1007/s11229-022-04015-z
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DOI: https://doi.org/10.1007/s11229-022-04015-z