Challenging Lewis’s challenge to the best system account of lawhood Draft. Final version forthcoming in Synthese. Rafal Urbaniak Centre for Logic and Philosophy of Science, Ghent University Institute of Philosophy, Sociology and Journalism, University of Gdansk https://ugent.academia.edu/RafalUrbaniak rfl.urbaniak@gmail.com Bert Leuridan Centre for Philosophical Psychology, University of Antwerp Centre for Logic and Philosophy of Science, Ghent University https://www.uantwerpen.be/en/staff/bert-leuridan/ bert.leuridan@uantwerpen.be November 23, 2016 Abstract David Lewis has formulated a well-known challenge to his Best System account of lawhood: the content of any system whatever can be formulated very simply if one allows for perverse choices of primitive vocabulary. We show that the challenge is not that dangerous, and that to account for it one need not invoke natural properties (Lewis, 1983) or relativized versions of the Best System account (Cohen and Callender, 2009). This way, we help to move towards a better Best System account. We discuss extensions of our strategy to the discussions about the indexicality of the notion of laws of nature (Roberts, 1999), and to another trivialization argument (Unterhuber, 2014). 1 Lewis’s challenge to the best system account of lawhood Lewis (1973, 1983) takes laws of nature to be those regularities which earn inclusion in the ideal system(s) to which we aspire in science. An ideal system has to strike a balance between simplicity and strength. 1 So, on his view, to be able to distinguish between accidental generalizations and real laws of nature, we need to be able to assess the simplicity and strength of a theory. This approach to the notion of lawhood is usually called the Mill-Ramsey-Lewis account of laws of nature (MRL, for short). 2 1 Thus: “I take a suitable system to be one that has the virtues we aspire to in our own theory-building, and that has them to the greatest extent possible given the way the world is. It must be entirely true; it must be closed under strict implication; it must be as simple in axiomatization as it can be without sacrificing too much information content; and it must have as much information content as it can have without sacrificing too much simplicity. A law is any regularity that earns inclusion in the ideal system. (Or, in case of ties, in every ideal system.)” (Lewis, 1983, 367) 2 Another well-known acronym is BSA (for Best System Account). 1