Davidson has argued that there can be no laws linking psychological states with physical states. I stress that this argument depends crucially on there being no purely psychological laws. All of this has to do with the holism and indeterminacy of the psychological domain. I criticize this claim by showing how Davidson misconstrues the role of ceteris paribus clauses in psychological explanation. Using a model of how ceteris paribus clauses operate derived from Lakatos, I argue that if Davidson is correct, then there can be no purely physical laws either. This is illustrated with a case from immunology involving interferons. Since there clearly are physical laws, Davidson cannot be correct.