Exclusion principle and the identity of indiscernibles: A response to Margenau's argument

Michela Massimi
University of Edinburgh
This paper concerns the question of whether Pauli's Exclusion Principle (EP) vindicates the contingent truth of Leibniz's Principle of the Identity of Indiscernibles (PII) for fermions as H. Weyl first suggested with the nomenclature ‘Pauli–Leibniz principle’. This claim has been challenged by a time-honoured argument, originally due to H. Margenau and further articulated and champione by other authors. According to this argument, the Exclusion Principle—far from vindicating Leibniz's principle—would refute it, since the same reduced state, viz. an improper mixture, can be assigned as a separate state to each fermion of a composite system in antisymmetric state. As a result, the two fermions do have the same monadic state-dependent properties and hence are indiscernibles. PII would then be refuted in its strong version (viz. for monadic properties). I shall argue that a misleading assumption underlies Margenau's argument: in the case of two fermions in antisymmetric state, no separate states should be invoked since the states of the two particles are entangled and the improper mixture—assigned to each fermion by reduction—cannot be taken as an ontologically separate state nor consequently as encoding monadic properties. I shall then conclude that the notion of monadic properties together with the strong version of PII are inapplicable to fermions in antisymmetric state and this undercuts Margenau's argument.
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DOI 10.1093/bjps/52.2.303
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Citations of this work BETA

Discerning Fermions.Simon Saunders & F. A. Muller - 2008 - British Journal for the Philosophy of Science 59 (3):499 - 548.
Physics and Leibniz's Principles.Simon Saunders - 2003 - In Katherine Brading & Elena Castellani (eds.), Symmetries in Physics: Philosophical Reflections. Cambridge University Press. pp. 289--307.
Discerning Elementary Particles.F. A. Muller & M. P. Seevinck - 2009 - Philosophy of Science 76 (2):179-200.
On Kinds of Indiscernibility in Logic and Metaphysics.Adam Caulton & Jeremy Butterfield - 2012 - British Journal for the Philosophy of Science 63 (1):27-84.

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