 A third notion of reductionism is more metaphysical in focus than either the conceptual or theoretical ideas reviewed so far. According to this notion, reductionism means that the properties expressed by the predicates of (say) a psychological theory are identical to the properties expressed by the predicates of (say) a neurobiological theory — in other words, this version of reductionism is in essence a version of type physicalism or the identity theory. As we have already seen, however, since a physicalism need not be a type physicalism, it need not be reductionism in this metaphysical sense either