UC San Diego

What theory along with its ontological commitments should we adopt? David Lewis and Nicholas Rescher offer competing theories for the interpretation of counterfactuals and modality involving statements. Using Quine's criterion for theory selection, this paper will argue that, when one weighs the various merits and difficulties of these two theories, it appears that a constructivist theory like Nicholas Rescher's offers a greater balance of theoretical advantages than does David Lewis' realist view. I begin by surveying the early attempts made by logicians to understand counterfactuals and modal statements and then proceed with a similar inquiry into a mathematician's efforts at interpreting mathematical statements. I try to show how similar ontological problems arise in the histories of both disciplines and how both disciplines first attempt to meet these difficulties by espousing a realist perspective, that of mathematical Platonism and David Lewis' possibilism. I then argue that a host of problems may be avoided and a number of metaphysical questions may be answered if we adopt a constructivist approach to modal and mathematical statements. With respect to mathematical statements, Philip Kitcher offers a constructivist theory which interprets mathematical statements as referring to idealized constructions of an ideal mind rather than to abstract Platonic entities. This paper concludes that we should adopt Rescher's actualist theory over David Lewis' possibilism and shift our ontological commitments from possible worlds to ideal constructions. Accordingly, modal statements are true in virtue of the constructions which the mind is ideally able to perform upon the actual physical world. Among its theoretical virtues, Rescher's theory explains the truth of counterfactuals in connection with ordinary experience, allows for the continued use of a uniform semantics for modal discourse, and avoids certain epistemological problems which rival theories face