Functionalism and the Metaphysics of Causal Exclusion

Take functionalism to be the thesis that mental property M is the property of having some other property that plays a certain characteristic causal role R.1 Functionalists are usually physicalists, and so take mental properties to be physically realized, such that for any mental property M, there’s a physical property P that fills R. Causal exclusion looms. Functionalism takes mental properties to be characterised by causal roles that are filled by something else. As M’s realizer, P must do all the causal work (whatever that is) that R involves, apparently leaving nothing for M to do. If mental properties are causally redundant, and if causal novelty is necessary for robust ontological commitment, then mental properties aren’t really real.2 This would be no small irony if true, because historically, functionalism was motivated by the need to explain how physically different creatures could be in the same mental state; small comfort to be told that it’s by dint of falling under predicates that don’t pick out genuine properties. Label properties that do causal work “causally efficacious”, setting aside for now the question of what causal work is. Those who argue that functionalism is consistent with the causal efficacy of mental properties typically adopt one of the following strategies: (i) deny that causal novelty is necessary for ontological commitment and argue that functional properties can inherit the efficacy of their realizers, resulting in a kind of causal over-determination;3 (ii) defend a difference-making theory of causation that entails that functional properties are causally novel after all.4 Proponents of (i) defend, or at least recognise the need to defend, theories of causation that vindicate their claim that functional properties inherit efficacy from their realizers. The task then is to argue that the kind of over-determination involved isn’t problematic.