Abstract

Glymour, Scheines, Spirtes, and Kelly argue for ‘Spearman's Principle’: one should (ceteris paribus) favour the theory whose ‘free parameters’ need assume no particular values for the theory to save the ‘constraints’ holding of the phenomena. I argue that the rationale they give for Spearman's Principle fails, but that (contra Cartwright) Spearman's Principle cannot be made to favour either of two theories depending on how they are expressed. I examine how one must motivate the demand for a scientific explanation of a parameter's value and how one justifies believing that a constraint should be explained independent of any parameter's particular value.