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Physicalism is the thesis that everything is either physical, or depends on the physical. It is usually taken to be committed to the causal closure of physics. But if physics is causally closed, then there can be no genuine causation from the entities, properties and processes of such ‘higher-level’ sciences as chemistry and biology ‘downwards’ to the physical. Arguments for the causal closure of physics have presented it as an empirical thesis, made plausible by the onward march of quantum-mechanical explanation. Some physicalists even cite this onward march as the historical explanation of why versions of emergentism which are incompatible with the causal closure of physics were so much less widespread after the emergence of quantum mechanics in the 1920s. Thus, for instance, C.D. Broad formulated an emergentist position according to which the possession of a chemical property by an object may confer on it causal capacities that transcend those it possesses in virtue of its physical properties. But, so the physicalist story goes, quantum-mechanical explanations of chemical structure and bonding were able to proceed without appeal to any such ‘downward causation.’ Hence chemical emergentism became much less plausible. In this paper, I investigate Broad’s characterisation of downward causation, and question whether it is as incompatible with modern quantum-mechanical explanation as contemporary physicalists think.
