Abstract
Physicalism is the view, roughly, that everything is physical. This thesis is often characterized in terms of a particular supervenience thesis. Central to this thesis is the idea of physical duplication. I argue that the standard way of understanding physical duplication leads—along with other claims—to a sub-optimal (and perhaps surprising) consequence for the physicalist. I block this consequence by shifting to an alternative sense of physical duplication. I then argue that physicalism is best characterized by a supervenience thesis that employs both the new sense of physical duplication and a new class of possible worlds.
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Notes
Friends of a posteriori necessities believe that the realm of metaphysically possible worlds is narrower than that of logically possible worlds, and so would distinguish metaphysical from logical possibility. To simplify matters, I will ignore this distinction in what follows. My arguments in no way depend on my doing so.
Chalmers notes that, on some accounts, the basic physical laws are determined by the totality of the particular physical facts. I will not deal with such accounts in this paper.
This closely resembles the understanding of physical duplication found in Melnyk (2003). For Melnyk, a world is a physical duplicate of our world if and only if it has (a) exactly the same distribution of physical tokens as our world and (b) exactly the same laws of physics as our world (p. 51).
Chalmers (1996) offers a different solution to the epiphenomenal ectoplasm problem. He takes the physicalist to claim that any physical duplicate of our world has all the positive facts of our world, as opposed to being a duplicate of our world simpliciter. This allows such worlds to have extra positive facts, including facts concerning epiphenomenal ectoplasm, angels, and ghosts. This solution does not make use of the notion of ‘minimal physical duplication.’
By a ‘mere’ nomological mental–physical supervenience thesis I mean to express the thesis that the mental nomologically supervenes on the physical, but does not supervene on the physical in any stronger sense.
It might be thought impossible for a minimal physical duplicate of our world to have non-physical laws, and thus to have basic psycho-physical laws. Interestingly, if this is correct, then the truth of naturalistic dualism would make a mere nomological mental–physical supervenience thesis (formulated in terms of ‘minimal physical duplication’) vacuously true, for no nomologically possible world could be a minimal physical duplicate of our world. But perhaps minimal physical duplication should be understood as ruling out only additional non-physical entities (e.g., ectoplasm and ghosts), not non-physical entities and non-physical laws. So understood, a minimal physical duplicate of our world could have basic psycho-physical laws. I do not think much rides on this point and so will not pursue it any further here. (Notice that this issue does not arise if Chalmers’ solution to the epiphenomenal ectoplasm problem is adopted.)
If one prefers Chalmers’ solution to the epiphenomenal ectoplasm problem over Jackson’s, then [Non-Basic Laws] can be altered as follows: if physicalism is true, then any possible world that is a physical duplicate of our world has all of our world’s non-basic laws (and perhaps other laws as well).
Similar reasoning, when combined with the previous footnote’s alternative formulation of [Non-Basic Laws], yields a formulation of [Consequence 1] that differs only in its omission of the word ‘minimal.’ Because a physical duplicate, w, of our world has all of our world’s basic physical laws and, according to the physicalist, non-basic laws, any additional laws that it might have must be compatible with our world’s laws. Consequently, all the laws of our world will hold in w, which is just to say that w is a nomologically possible world.
A metaphysical S-physical supervenience thesis and a nomological S-physical supervenience thesis correspond to one another if and only if they replace the variable ‘S’ with the same set of properties.
I will offer a guess as to why this is so, assuming that it is, in Sect. 3.
Recall that for w to be a minimal physical duplicate of our world, w must have our world’s basic physical laws.
According to Wikipedia, this is the Burj Khalifa skyscraper in Dubai.
It might be thought that some possible worlds that have different basic physical laws than our world will be unable, in virtue of this difference, to be duplicates of our world with respect to the instantiations of physical properties and relations. This seems plausible to me. My argument, though, only requires that some possible worlds that have different basic physical laws than our world are duplicates of our world with respect to the instantiations of physical properties and relations.
A mere nomological supervenience thesis relating mousetraps to the physical will be false since such a thesis denies that mousetraps metaphysically supervene on the physical.
Minimal Physical Duplication’ (def): a possible individual y is a minimal physical duplicate of an individual x in our world if and only if y (i) has all the same instantiations of physical properties and relations as x, (ii) is governed by all of the same basic physical laws as x, and (iii) has nothing more.
[Non-Basic Laws’] If physicalism is true, then any possible world that has our world’s basic physical laws has our world’s non-basic laws (and possibly other laws, basic or non-basic). This principle is stronger because it concerns all possible worlds that have our world’s basic physical laws, not merely those possible worlds that are minimal physical duplicates of our world. [Non-Basic Laws’] is certainly more contentious than its weaker counterpart.
To illustrate, the analogue of [Consequence 1] is the following: if physicalism is true, then any possible individual that is a minimal physical duplicate of an individual in our world is a nomologically possible individual (i.e., is an individual who resides in a nomologically possible world).
An anonymous referee points out that if, for example, inhabiting a world in which the laws of thermodynamics hold, counts as a physical property, then a possible world that is a minimal physical duplicate* of our world will consequently be a world where the laws of thermodynamics hold, in which case I cannot separate (as I wish to do) duplication of instantiations of physical properties and relations from duplication of basic physical laws. However, recall from Sect. 1 that physical duplication (unstarred) is standardly formulated in terms of both the duplication of instantiated physical properties and relations, on the one hand, and the duplication of basic physical laws, on the other hand. Given this, there appears to be implicit in the literature on supervenience and physicalism an understanding of physical property on which facts about the laws governing a world (or laws governing an entity in a world) do not count as physical properties of the world. Surely, more needs to be said on this topic. However, it seems that since I am using ‘physical property’ in the standard way, this task need not be undertaken in this paper.
This should be understood as including the empty set, for perhaps a possible world with all the same instantiations of physical properties and relations as our world can have no basic physical laws. I thank an anonymous referee for making this point.
Recall that [Metaphysical S-Physical Supervenience] is true when instantiated with both kinds of properties.
See footnote eight for a complication.
Identity theorists should adopt the former, non-reductive physicalists should adopt the latter.
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Acknowledgments
I would like to thank William Roche, Lawrence Shapiro, Alan Sidelle, and Danielle Wylie for their comments on an early draft of this paper and for their willingness to discuss the topic with me at length. Alan was especially helpful and generous at the early stages of this project. Any errors, of course, should be attributed to me, not them. Thanks also to two anonymous referees at this journal for their very helpful comments and advice during the review process.
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Roche, M. Physicalism and Supervenience: A Case for a New Sense of Physical Duplication. Erkenn 81, 669–681 (2016). https://doi.org/10.1007/s10670-015-9761-y
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DOI: https://doi.org/10.1007/s10670-015-9761-y