Prospects for Strong Emergence in Chemistry
Robin F. Hendry
How is it possible that there could be downward causation in chemistry? Since the chemical revolution chemists have pursued a research programme which has successively identified the elemental constitution of compound substances, and explained their behaviour in terms of that constitution. In the nineteenth century they began to think of elemental composition in terms of atomic constitution, and devised structures at the atomic scale, diversity among which accounted for the existence of distinct substances (isomers) that have the same elemental composition. In the twentieth century the relationship was deepened further by discovering the structure of atoms themselves, and how their parts (electrons and nuclei), and the interactions between them, underpin the structures that individuate substances and explain their behaviour. At the same time the structures themselves were fleshed out using the joint resources of theory and experiment. Thus classical mechanics, the structural theories of the nineteenth century, the ‘old’ quantum theory, quantum mechanics, X-ray crystallography, spectroscopy, and dear old chemical inference all pulled together to provide, by the early twenty-first century, detailed theories of how nuclei and electrons are arranged within substances, how they move and interact, and how these structures and processes give rise to the phenomena that chemists and physicists study.
Does this undoubted intellectual achievement not amount to a reduction? It does not, I argue. Chemistry supplied the atoms, and initially the physicists took some persuading of their existence: physicists came late to chemical atomism.1 Furthermore, chemistry’s distinct perspective on structure at the molecular scale was an indispensable part of the development of structural explanation in both physics and chemistry. The whole enterprise was a collaboration, and it is perhaps surprising that it is not seen that way more widely in philosophy and physics, and indeed chemistry itself. We should regard the process as a synthesis of chemistry and physics, not a reduction of one to the other. But the reductionist will say, quite correctly, that these are merely historical points.
We have a theory of everything (non-relativistic quantum mechanics), at least for chemistry, that can, in principle, explain everything about molecules. Perhaps so, but let’s examine the assumption that such a theory exists, and if it does exist, then let us also examine what this theory can, and cannot, do for us.