On the Causal Completeness of Physics

Abstract

According to an increasing number of authors, the best, if not the only, argument in favour of physicalism is the so‐called ‘overdetermination argument’. This argument, if sound, establishes that all the entities that enter into causal interactions with the physical world are physical. One key premise in the overdetermination argument is the principle of the causal closure of the physical world, said to be supported by contemporary physics. In this paper, I examine various ways in which physics may support the principle, either as a methodological guide or as depending on some other laws and principles of physics.

Acknowledgements

I am grateful to Ulises Moulines and Dan López de Sá, for their comments on previous versions of this paper, and to Michael Maudsley, for his grammatical corrections. I have also benefited from conversations with the physicist Mariano Santander, and especially from some very helpful comments from two anonymous referees and the editor of ISPS, James W. McAllister. The paper has significantly improved thanks to them. This work is funded by the Research Projects HUM2005‐03211/FISO and HUM2005‐07539‐C02‐00.

Notes

[1] Some examples: Peacocke (1979), Papineau (1990, 2001), Antony and Levine (1997), Sturgeon (1998), and Levine (2001).

[2] This positivist inspiration for physicalism can again be found in some contemporary authors, who defend new versions of identity theories, such as Block and Stanaker (1999) and Polger (2004). According to this view, an identity theory is supported by an inference to the best explanation argument. Furthermore, authors such as Devitt (1996), who insist on the importance of ‘what is F?’ questions, can be read as requiring physicalism from epistemic needs. All these authors, however, have to face the so‐called ‘problem of multiple realizability’, which in its most neutral version holds that, as a matter of fact, scientific taxonomies do not fit the reduction schema, for the conceptual cuts of special sciences correspond to a disjunction of concepts of physics.

[3] I should stress that I do not want to be committed to a rejection of the unificatory programme. My intention here is to analyse the strength of the overdetermination argument (of one of its premises, in fact) under the assumption that it is at least more convincing than the classical arguments for reductionism.

[4] Physicalism is usually stated by means of a supervenience thesis, according to which any possible world that is a minimal physical duplicate of the actual world is indiscernible from it in all respects (see Jackson 1998).

[5] By ‘physical entities’, I mean ‘entities postulated by a true theory of physics’. There is a problem with this understanding of physicalism, due to our ignorance of this true theory and therefore of what entities we are in fact speaking about (see Crane and Mellor 1990). I will just assume that there is a solution to this problem, and that the true theory of physics will not include mental (or, in general, dubious) concepts in its repertoire. Another thing worth mentioning is that physicalism as I will understand it here speaks about physics ‘considered strictly’, and not about a broad construction of ‘the physical’. The same goes for the term ‘physical world’: such a world is the world as depicted by the true theory of physics.

[6] Besides other possible considerations, there is at least a reasonable doubt as to the status of uniqueness of the argument from overdetermination, which is that some deeply committed physicalists (or, rather, naturalists, but the overdetermination argument is also presented as the argument for naturalism; see Antony and Levine 1997) take it that mental states are not causally efficacious. For instance, Millikan says that ‘on the account of this essay, the semantic category of a thought is determined relative to its biological functions, which depend in turn upon its history, upon its place relative to certain prior events. But having a certain history is not, of course, an attribute that has “causal powers”’ (Millikan 1993, 136).

[7] This label is not very fortunate, since there are what one would call ‘symbolic generalizations’ that do not look like methodological guides (equations, for instance), and also there are methodological guides that one would not call ‘symbolic generalizations’: Kuhn's own example of the action‐reaction law is one of these. For this reason, I will switch to the label ‘guiding principles’, used in the structuralist literature with roughly the same intended meaning as Kuhnian generalizations. This terminological change, as well as the idea that the CCP could be considered one of these guiding principles, was suggested to me by Ulises Moulines.

[8] From the belief in the CCP (plus the belief in mental causation), Epicureans inferred the material nature of the soul. Leibniz, however, did not deduce the physical nature of the mental from the principle. As he believed in the non‐physical nature of the mind, he opted for parallelism.

[9] The talk of ‘downward causation’ implies that physics is the bottom‐level science and that all supervenes one way or another on the physical. However, this may be a controversial view, as will be explained.

[10] See also the essays contained in Andersen et al. (2000). There, Moreno and Unmerez, for instance, defend emergentism at the cellular level. El‐Hani and Pereira (1999) make much the same point as El‐Hani and Emmeche (2000).

[11] Van Gulick says that brains are ‘self‐sustaining or self‐reproductive in the face of perturbing physical forces that might degrade or destroy them’. Also, ‘it is because of the existence and persistence of the pattern that the particular constituents of its instances were recruited and organized as they are’ (Van Gulick 1993, 252).

[12] At the present stage, I can only say that I have some doubts that the ‘basketball team model’ may be applicable to non‐intentional entities and that I also doubt that there is a notion of causality that can be used to explain this kind of ‘downward causation’. It is a topic, in any case, that must wait for another occasion. In the present context, I only mention it in order to show that there are authors who discuss the inductive record usually assumed to be favourable to the CCP. Incidentally, this does not mean that I think that it is not possible to found our belief in the CCP in scientific induction. As I have said, and will say again later, it appears to be a promising route, only that (a) the historical record suggests the approach of conservation laws and (b) this latter approach makes it possible to give a definite closed sense to the principle by which we can exclude the intervention of non‐physical causes in the physical world.

[13] On transference, see Fair (1979), on transmission, Salmon (1984; 1997), and on exchange, Dowe (2000a). In what follows, I will use all these terms as if they were synonymous. They express clearly different notions, but I do not think these differences are relevant for our present purposes.

[14] Let us assume, for the time being, that neurophysiology and biochemistry are reducible to physics.

[15] A very brief hint: Fred Dretske (1988) holds that mental events are the structuring causes of behaviour, while neurophysiological events trigger it. That is, mental and neurophysiological events are different kinds of causes, and so they do not compete: mental events as structuring causes are responsible for the fact that neurophysiological events trigger behaviours. This is a possible non‐reductivist solution to Malcolm's problem. Yet, it is unlikely that Dretske's account may provide a non‐reductivist solution to the problem of mental causation if, instead of (a'), we have the original causal closure principle, for in that case it seems that the principle (arguably) establishes that structuring causes must be physical.

[16] Although we would still have to show that neurophysiology and biochemistry are reducible to physics.

[17] One can say that in such a case, the first body has the second's energy, and vice versa. This would be allowed by Fair's notion of transference, but not by more empiricist at‐at accounts (see Salmon 1997).

[18] There is no reason why the physicalist could not complement energy with charge, momentum, or any other physically conserved quantity.

[19] Not at least to Fair—see below.

[20] Perhaps our world turns out to be one of them. On the modal implications of CQ theories, see Kistler (1997), Dowe (2000b), Vicente (2002). There are several other, apparently severe, problems with Dowe's individuation of objects.

[21] From now on, I will mainly deal with energy. It must be kept in mind, however, that physical effects are not restricted to variations in energy.

[22] As a referee has pointed out, this is as it should be, given that, at least in classical physics, forces and energies are interderivable (for instance, force is the negative spatial derivative of potential energy). Yet, it seems to me that the ‘sound’ of the anti‐physicalist positions is not the same. It is one thing to say that there might be non‐physical conservative forces and another to suggest that there are local interactions where, contrary to received wisdom, energy is not conserved. To my ears, the first claim, or, rather, the first way of putting things, sounds less committal. But this may be subjective, and in any case I want to stress that it is a question about ‘sounds’ (colouring, as some would say).

[23] Against this idea, it can be said that Cartesian mechanics held that there was a property that was universally conserved, quantity of movement, yet Cartesianism also held that the mental substance brought about physical changes. In the Cartesian framework, what the mind could do is change the direction of the moving particles (see Lowe 2000 and Papineau 2001 on this point). This fact could make us qualify our view: not just any physical theory that holds that some property is conserved holds thereby that the physical world is causally closed. But it is not clear that Cartesianism counts against the view presented here rather than confirms it, for in a theory such as Cartesian mechanics, physical causal interactions cannot be the exchange of conserved quantities either. The change in direction of a movement is a physical effect that does not result from such an exchange (one does not have to think of non‐mechanical interactions or forces: it is enough to think of an elastic interaction of two bodies of equal mass and velocity colliding elastically with the same direction. There is a physical effect of this collision, namely, the change in the sense that each of the bodies follows, but it is not the result of the exchange of their quantities of motion).

[24] The link with conservation laws is made explicit under the CQ approach: simply, there cannot be any physical effects if there are no physical conserved quantities. But the co‐variation between conservation laws and the belief in the CCP should be explained in a different, neutral, way. My hypothesis is that conservation laws block the ‘easy response’—there are causes that create energy, momentum, or whatever. They force the anti‐physicalist to defend the existence either of non‐physical conservative forces or of odd forms of energy, etc., transformable into energy, etc., and this looks like a harder route.

Additional information

Notes on contributors

Agustín Vicente

Agustín Vicente Assistant Professor is at University of Valladolid, Spain.