– The basic idea for supervenience is that there is no change in the mental (M) unless there is a change in the physical (P). Put differently, no mental activity without physical activity – the mental supervenes on the physical.
– What supervenience attempts to secure, generally speaking, is a non-reductive physicalist account of the mind – ie, this isn’t a simple identity theory (mind = brain) or an eliminative theory. In other words, it’s a theory of the mind that aims at an account of the mind that, while not reducing the mental to the physical, shows the mental to not be independent of the physical.
– There are various accounts of supervenience, but generally it’s accepted that it must be of the strong kind to be a really physicalist theory of the mind. This means, roughly, that in no nomologically possible world (this move is made so as to preclude the metaphysical possibility of, say, dualism) :
‘Necessarily, for each property M in M, if anything x has M, then there is a property P in P such that x has P, and necessarily if anything has P it has M.’ (‘A Companion to the Philosophy of Mind’, p. 579)
– As Jaegwon Kim notes, its difficult to see exactly how this isn’t a reductive theory – if for M it’s necessary for P, there it certainly seems like M entails P. Kim goes so far as to declare that supervenience won’t give up a nonreductive theory of mind, in fact.
– A further difficulty (and the above is a very crude sketch of one objection) comes when the issue of causation comes into play – Kim appropriately dubs his theory of causation ‘supervenient causation’ – M causes P because M supervenes on P. Kim holds to a theory of ‘causal explanatory exclusion’, or the doctrine that there is at most one full, complete causal explanation for a given event, and this, coupled with his principle of ‘causal closure’ – any physical event that has a cause at time t has a physical cause at t – seems to really nix any idea of mental causation in the bud, which is precisely what Kim is trying to hold on to (the spectre of epiphenomenalism is always hovering nearby). If M causes P, and M supervenes on P, and M really is distinct from P, then it would appear that we have two causes of P – M and P. But given Kim’s commitment to the causal priority of P, what causal role is there really for M? It seems that epiphenomenalism has been invited in here.