Some philosophers have suggested that, instead of attempting to arrive at a satisfactory definition of the physical, we should adopt the ‘via negativa.’ That is, we should take the notion of the mental as fundamental, and define the physical in contrast, as the non-mental. I defend a variant of this approach, based on some information about how children form concepts. I suggest we are hard-wired to form a concept of intentional agency from a very young age, and so there’s some reason to believe that our concept of the physical does include, as part of its content, a contrast with the mental.