Nagel is not a physicalist because he does not believe that an internal understanding of mental concepts shows them to have the kind of hidden essence that underpins a scientific identity in, say, chemistry. But his skepticism is about current physics: he envisages in his most recent work that people may be close to a scientific breakthrough in identifying an underlying essence that is neither physical (as people currently think of the physical), nor functional, nor mental, but such that it necessitates all three of these ways in which the mind "appears" to us. The difference between the kind of explanation he rejects and those that he accepts depends on his understanding of transparency: from his earliest paper to his most recent Nagel has always insisted that a prior context is required to make identity statements plausible, intelligible and transparent.