Extensionalism, naturalism, and probability: can Quine’s anti- modalism survive quantum mechanics?

This thesis highlights a major fault in Quine's philosophy. I argue that Quine's combination of extensionalism and naturalism cannot be maintained due to the role played by probability in Quantum Mechanics (QM), and that attempts to save Quine's perspective fail, requiring Quine to drop either extensionalism or naturalism.

Chapter I examines Quine's perspective on modality and retrieves the few passages where probability is involved. I consider Quine's treatment of dispositions and underline that probability is intended by Quine either as subjective degrees of belief or, in the case of QM, as propensity, where this is still thought by Quine to be extensional.

In Chapter II, I make preliminary remarks about the available interpretations of probability considering that the majority of them cannot be used to make sense of QM probability in the Quinean system.

In the third chapter, after having considered the interpretations of QM, I examine Heisenberg's potentialities that ground his understanding of probability in QM. I argue that potentialities as conceived by Heisenberg cannot be understood in any extensionalist way. Relying on this, I propose two scenarios from QM where de re modality is involved.

Chapter IV presents a potential solution for Quine involving Lewis' Best System account of laws of nature. I underline that even if Lewis' system proposes a clear extensional framework it presents major drawbacks when it comes to combining it with Quine's naturalism.

The discussion on potential solutions to preserve Quine’s account continues in Chapter V, where I consider Quantum Bayesianism (QBism) that endorses subjective probability in QM. Also in this case, major discrepancies with the Quinean perspective are found with particular reference to the role of the Born Rule.

On this basis, the solution of the flaw appraised in Quine's systems might involve the abandonment of either extensionalism or naturalism.