Abstract
The issue whether or not the most fundamental entities described by non-relativistic quantum mechanics are individual objects is of interest for both philosophers and metaphyiscians, and has received a great deal of attention lately. Even though there is no universal consensus, the most popular view seems to be that quantum particles are not individual objects. This paper offers a critical analysis of recent arguments in support of this position, and a defence of the more traditional alternative.
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Notes
- 1.
Meaning, basically, that it is a purely metaphysical posit that is not directly causally efficacious and, therefore, cannot be known in itself. Whether this also means that it should be left out of the domain of what we conjecture to be ‘real’ on the basis of our best science is, as we will see, more controversial than is normally believed.
- 2.
Muller, Saunders and Seevinck start from single-particle operators whose physical significance is quite uncontroversial, and go on to construct relations out of (some of) the corresponding projectors and their possible values.
- 3.
That is, if one endorses the ‘neutral’ definition of individuality above, and rejects Ladyman and Bigaj’s request that metaphysical discernibility correspond to the availability of physical procedures to actually distinguish.
- 4.
The contextualist take on identity supported by Ladyman was endorsed earlier by Stachel (see, for instance, Stachel [2004]), who first developed it in the context of a discussion of General Relativity and the ontology of space-time. Like Ladyman, Stachel moved in his work from initial “local’ claims and results to the defence of contextualism as a thesis of ‘global’ validity.
- 5.
Clearly, this is relevant even beyond the context of the discussion of the ontological status of quantum particles, as it is connected to important general questions concerning the methodology one should implement when exploring issues at the boundary between empirical science and metaphysics.
- 6.
Recall, in this connection, that in their proofs Muller, Saunders and Seevinck need to assume the bare numerical difference grounding the countability of quantum particles. Although, as argued earlier, it is perfectly legitimate to do so, one may instead take countability to have a direct ontological import, that is, to justify talk of individuality even independently of considerations concerning (in-)discernibility.
- 7.
Scotus certainly intended haecceitates as full-blown metaphysical components of individuals.
- 8.
Incidentally, the metaphysically deflationary reading of primitive intrinsic identity and individuality also goes back to the Scholastic tradition, although to the ramification of it shaped by Ockham’s nominalism rather than by Scotus’ realism. Ockham surely did not intend haecceitates as ontological additions to objects and properties.
- 9.
Teller’s claims to this effect are not completely uncontroversial, but it is at least fair to claim that skepticism about counterpart theory may lead one to consider this option unappealing anyway.
- 10.
These correlations can be taken as categorical irreducible relations, as suggested for instance by Muller, Saunders and Seevinck, but also as (monadic or relational) dispositions for measurement outcomes. Which option is to be preferred doesn’t matter here.
- 11.
It may be objected that one should presuppose supervenience whenever possible, and so non-factorisable entangled states should be deemed ‘metaphysically special’. However, (1) I think the explanatory efficacy of a presupposition to the effect that (A) fails in general, and in particular the fact that such a presupposition allows one to stick to common sense at a more important level (that of the ontological status of particles and larger physical systems) suffices for resisting the objection; moreover, (2) it is possible to implement criteria for evaluating the non-reducibility of global to local properties that make quantum non-supervenience independent of, and non-reducible to, entanglement and non-factorisability: Seevinck (2004), for instance, proposes one such criterion, based on the local resources available to agents for individuating well-defined monadic properties (so suggesting a shift from an ontological to an epistemological justification for claims of (non-)supervenience and holism).
- 12.
This might be restricted to systems of identical particles, but whether or not one does so is not crucial here.
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Morganti, M. (2012). Identity in Physics: Properties, Statistics and the (Non-)Individuality of Quantum Particles. In: de Regt, H., Hartmann, S., Okasha, S. (eds) EPSA Philosophy of Science: Amsterdam 2009. The European Philosophy of Science Association Proceedings, vol 1. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-2404-4_20
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