In response to the semantic objection, Freud must show that the concept "unconscious mental state" is at least conceptually coherent. Preferably, Freud should start from a definition of "mental" that is shared with his Cartesian opponents to provide a common basis for discussion. To identify what form such a definition might take, I explore five prominent theories of the nature of concepts, arguing that Freud is best understood as applying what I call a "black box essentialist" account: A state is mental if and only if it possesses the same mental-relevant essential property possessed by the base set of conscious mental states. This definition allowed Freud to reject the semantic objection and focus instead on the theoretical issue of the essence of the mental.