Is Kantian Projectivism the Only Hope for Grounding the Principal Principle? (No.).

Kant saw science as presupposing that the natural laws bring maximal diversity under maximal unity. Many philosophers, such as David Lewis, have regarded objective chances as upshots of science's aim at systematic unity--as ideal credences projected onto the world. This Kantian projectivism has seemed the only possible way to account for the rational constraint (codified by the 'Principal Principle') that our credences about chances impose on our credences regarding what they are chances of. This paper examines three ways of elaborating Lewis's Kantian strategy for explaining this rational constraint. After arguing that none of these three approaches is unproblematic, the paper proposes a non-Kantian alternative account according to which a chance measures the strength of a causal tendency. [ABSTRACT FROM AUTHOR]