Two Kinds of Determinism

Lugano Philosophy Colloquia. Cristian Mariani

Physicists often speak as though determinism and determinacy were the same issue, yet the two questions are conceptually distinct. Determinism concerns the dynamics of a physical system: given its complete state at a time, is there exactly one way it can evolve, or can history fork into multiple possible futures? Determinacy, by contrast, concerns the ontology of the system at a time: does each of its properties have a single, sharp value, or can properties be irreducibly "fuzzy"? Standard characterizations of determinism (following Hoefer, Reichenbach, Peirce, and Glymour) require a "specified way things are" at a time, and this specification can be read either maximally—covering every physical quantity—or as requiring that quantities take determinate values. On this reading, determinism simply presupposes determinacy: without sharp values, the notion of a uniquely fixed future becomes unintelligible. Earman resists this, arguing that determinism can be formulated over set- or interval-valued magnitudes, so that determinacy is not strictly required. Examining Bohmian mechanics as a test case—where positions are always determinate and the guiding equation deterministic—suggests that in the theories we actually have, determinism and determinacy travel together, even if it remains unclear what a "minimal amount of determinateness" short of full determinacy could mean. The tentative conclusion is that, contrary to the received view, determinism and determinacy are not straightforwardly independent notions once we look past their conceptual definitions to the physical theories in which they are embedded.