Richard Andrew Healey

Although there has been some discussion in the literature of Bas van Fraassen's modal interpretation of Quantum Mechanics, it has for the most part been concentrated on difficulties that van Fraassen's viewpoint shares with those of some other authors, including Kochen, Dieks, and Healey. van Fraassen's approach has, however, some problems of its own; in this note we want to focus on what seems to us to be one of the most serious of these. The difficulty concerns immediately repeated non-disturbing measurements of the same observable on a single system. As is well known, von Neumann's Projection Postulate guarantees that such measurements will always give the same outcome; likewise, in the approaches of the “modalists” mentioned above, such ‘consilience of repeated measurements’ is in one way or another built into the formalism. By contrast, we shall argue, van Fraassen's modal interpretation neither guarantees this result nor adequately explains why it is unnecessary to do so.

The paradox of Wigner’s friend challenges the objectivity of quantum theory. A pragmatist interpretation can meet this challenge by judicious appeal to decoherence. Quantum theory provides situated agents with resources for predicting and explaining what happens in the physical world—not conscious observations of it. Even in bizarre Wigner’s friend scenarios, differently situated agents agree on the objective content of physical magnitude statements while, normally, quantum Darwinism permits agents equal observational access to their truth. Quantum theory has nothing to say about conscious experiences. But it does prompt us to reexamine the significance of everyday claims about the physical world.