The problem of extras and the contingency of physicalism

Perhaps all concrete phenomena obtain solely in virtue of physical phenomena. Even so, it seems that the world could have been otherwise. It seems that physicalism, if true, is contingently true. In fact, many believe that the actual truth of physicalism allows metaphysically possible worlds duplicating the actual world in all physical respects while containing immaterial extras, e.g. ghosts, spirits, or Cartesian souls, that no physicalist would believe actually exist. Here I focus on physicalism regarding mentality and argue that the doctrine does not allow possible worlds that physically duplicate the actual world while differing mentally. By revealing what physicalism (regarding the mind) does not allow, this essay helps us get clear on what the view really amounts to and why it is contingent.