My aim in this paper is to defend a version of the brain process theory, or identity thesis, which differs in one important respect from the theory put forward by J.J.C. Smart. I shall argue that although the sensations which a person experiences are, as a matter of contingent fact, brain processes, nonetheless there are facts about sensations which cannot be described or understood in terms of any physical theory. These 'mental' facts cannot be described by physics for the simple (...) reason that physical descriptions are designed specifically to avoid mentioning such facts. Thus in giving a physical explanation of a sensation we necessarily describe and render intelligible that sensation only as a physical process, and not also as a sensation. If we are to describe and render intelligible a person's sensations, or inner experiences, as sensations, and not as physical processes occurring in that person's brain, then we must employ a kind of description that connot be derived from any set of physical statements. (shrink)