The Causal Argument for Physicalism

Physicalism can be thought of as the view that the mental is “nothing over and above” the physical. I defend a formulation of this view based on supervenience. Physicalism may be supported in two ways: either by providing an explanatory account of the mind in physical terms, or by philosophical argument. Since we have only a rudimentary scientific understanding of the mind, physicalism needs argument. The most promising such argument is the causal argument, which may be summarised thus: (i) mental properties have physical effects; (ii) physics is causally complete (all physical effects have physical causes); (iii) effects are not generally overdetermined; so (iv) mental properties are physical. Of these premises, (i) relies on common-sense, (ii) relies on empirical support, and (iii) is a priori. I consider the merits of this argument by articulating two kinds of mental property emergence, ‘weak’ and ‘strong’, both of which are incompatible with physicalism. I show that the premises of the causal argument are compatible with weak emergence, and that the argument is therefore not deductively valid. The causal argument establishes that one of physicalism or weak emergence is true. However, weak emergence is problematic in ways that physicalism is not. If these problems are serious, then physicalism is to be preferred on other grounds, such as theoretical elegance and simplicity. However, I proceed to show that the soundness of the argument is questionable, as premise (ii) is unsupported by the available evidence. Strong emergence is inconsistent with (ii), so evidence for (ii) must (on reasonable assumption) be evidence against strong emergence. But all currently available evidence is consistent with strong emergence, and so this evidence does not support (ii). Future evidence might, but I argue that such evidence would need to involve the kind of scientific account of mind the lack of which motivates the causal argument in the first place. A well-supported causal argument, by the nature of the justification necessary for (ii), is otiose.