Do Dispositions and Propensities Have a Role in the Ontology of Quantum Mechanics? Some Critical Remarks

In order to tackle the question posed by the title – notoriously answered in the positive, among others, by Heisenberg, Margenau, Popper and Redhead – I first discuss some attempts at distinguishing dispositional from non-dispositional properties, and then relate the distinction to the formalism of quantum mechanics. Since any answer to the question titling the paper must be interpretationdependent, I review some of the main interpretations of quantum mechanics in order to argue that the ontology of theories regarding “wave collapse” as a genuine physical process could be interpreted as being irreducibly dispositional. In non-collapse interpretations, on the contrary, the appeal to dispositions is simply a way to reformulate the predictive content of the algorithm of the theory in a fancier metaphysical language. 1. Dispositions and the interpretive task of quantum mechanics In trying to understand the role of propensities or dispositions, if any, in the interpretations of quantum mechanics (henceforth QM), I think that one can do no better than start from a fundamental question once posed by John S. Bell: “What are quantum probabilities probabilities of?” As I see it, this question addresses two deeply related issues, both of which are relevant to evaluate the role of dispositions in QM. The first is an ontological question, namely an attempt to connect the formal structure of quantum theory with entities in the physical world, in order to try to figure out what the theory is about. I take it that, in all generality, interpreting the mathematical formalism featuring in physical theories ought to mean: 1.1 understanding the ontological implications of physical theories (“the scientific image”); 1.2 connecting the postulated ontology with our pre-theoretical experience of the world (“the manifest image”). ∗ I thank the audience in Madrid for helpful questions and criticism. In addition, I am highly indebted to Roman Frigg, Carl Hoefer and Federico Laudisa for their critical comments to a previous version of this paper, which significantly improved the final result.