 The status of arguments for physicalism and the supervenience of everything on the physical are contentious within metaphysics proper, but beyond the scope of this entry. Yet we think that the failure of chemical theory to be fully derivable from physics raises interesting questions about the doctrine of physicalism. Minimally, it points to longstanding worries that the domain of the physical is not well-defined. If chemical entities such as molecules and ions end up being part of the physical ontology, one might argue that this was not a case of the reduction of chemistry to physics at all but simply the expansion of the ontology of physics to encompass the ontology of chemistry.