How should ‘the physical’ be defined for the purpose of formulating physicalism? In this paper I defend a version of the via negativa according to which a property is physical just in case it is neither fundamentally mental nor possibly realized by a fundamentally mental property. The guiding idea is that physicalism requires functionalism, and thus that being a type identity theorist requires being a realizer-functionalist. In §1 I motivate my approach partly by arguing against Jessica Wilson's no fundamental mentality constraint. In §2 I set out my preferred definition of ‘the physical’ and make my case that physicalism requires functionalism. In §3 I defend my proposal by attacking the leading alternative account of ‘the physical,’ the theory-based conception. Finally, in §4 I draw on my definition, together with Jaegwon Kim's account of intertheoretic reduction, to defend the controversial view that physicalism requires a priori physicalism.