Abstract
Parfit’s new two-volume book, On What Matters, is used to make three points about the use of thought experiments in philosophy: thought experiments must be relevantly focused, finely-grained in descriptive force, and, when they meet these conditions, a thought experiment can overturn a substantial philosophical position, namely Parfit’s rejection of libertarian freedom. While principally a contribution to philosophical methodology, along the way this paper defends moral realism, Parfit’s rejection of a combined egoist and contractualist ethic, and a defense of libertarian free agency.