Propositional counter-factual definiteness and the EPR paradox

Abstract: In an empirical logic, an experimentally verifiable proposition P relating to a quantum system is assigned the value of either true of false if the system is in the pure state that belongs or, respectively, does not belong to the Hilbert subspace that represents P. Determined in such a way truth or falsity of P can be termed a factual truth-value of P. In the present paper, it is proposed to consider a counter-factual truth-value of P, i.e., either of the values, true or false, that would have been taken by P if the system had been in a pure state belonging to a Hilbert subspace that does not represent P. The assumption that it is always possible to speak meaningfully of counter-factual truth-values of experimental propositions can be called the hypothesis of propositional counter-factual definiteness. As it is shown in the paper, this hypothesis lies at the basis of the EPR paradox, a striking and influential thought experiment intended to defy predictions of quantum mechanics, such as one that measurements of spin along the different axes are incompatible. The purpose of this paper is to show that this hypothesis can be falsified by declining to paste together invariant-subspace lattices of contexts associated with the system (in other words, Boolean algebras or blocks) into one Hilbert lattice. Without such pasting, the EPR paradoxical inference cannot be reached.