It is a great pleasure to contribute to a festschrift for Hugh Mellor. In his articles, books and conversation, he has been one of the most significant influences on my thought.
In a rather too frequently discussed paper - as Mellor would be the first to hold - Crane and Mellor inveigh against physicalism. The common argument offered in favour of physicalism is based on the claim that the physical world is causally closed or, more specifically, that nothing non-physical causes something physical. In response, Crane and Mellor write:
Our mental states, intentional and otherwise, could - and would - affect our brain states and bodily movements even if the laws of physics made them all determined also by earlier brain states. The claim that a system thus constrained by non-mental laws must be closed, in the sense of being unaffectable by its mental states, simply does not follow - and it is not true.
(Crane and Mellor 1990:100)
In his later work, Mellor is even more explicit about what he has in mind. He writes:
overdetermination exists … the fact, if it is a fact, that a mental cause C which neither is nor supervenes upon a physical cause C′ of the same effect E would overdetermine E is no reason to deny that C is as effective a cause of E as C′ is.
Nevertheless, although Crane and Mellor are willing to countenance over-determination - even systematic overdetermination - in discussing the causal history of mental events and behaviour, many are not. That is the major reason why most philosophers of mind have become physicalists. A doughty few have tried to square their commitment to the causal closure of the physical world with their conviction that some mental properties are non-physical. These are the 'epiphenomenalists' (Campbell 1970:124-6; Jackson 1998:58; and, in some moods, Chalmers 1996:150-1, 191-203).