Many twentieth century philosophers have argued that we should reject ‘dualism’ and follow some form of ‘physicalism’. Strangely, the same philosophers often reject panpsychism (the view that everything has a mental aspect). Rejection of panpsychism seems itself to imply a dualism, since it means that some parts of the universe have mental aspects, whatever they might be, and others do not.
My impression is that the whole debate about dualism trivializes the key issues that the philosopher scientists of the seventeenth century tried to explain to us. There are all sorts of dualities within modern physics. Even the most despised ‘substance dualism’ of Descartes is probably a better insight into the way physics works than the simplistic versions clung to by modern ‘physicalists’. Attempts to avoid the label of dualism by proposing a ‘dual-aspect monism’ I suspect achieve little other than further confusion. Dualism means all sorts of different things in different contexts and, like most –isms, is probably best just ignored.
The following are some of the dualities within physics.
1. The job of physics is to explain our experiences in terms of patterns of change. Experience and change form a duality at the heart of physics. Physics has never provided an indication of some sort of ‘solid stuff’ defined in a way independent of experience. It describes the world entirely in terms of the mathematical rules that govern changes and how they link together in a way that generates experiences in a predictable fashion.
2. Modern physics divides dynamic units into two sorts, those that exclude other similar units from their domains (fermions) and those that do not (bosons). This is essentially the difference that Descartes noted between ‘extended matter’ and ‘thinking things’. If thoughts belong to bosons he would seem to have been right. Descartes rarely refers to physics and when he does he seems to use the word to mean the practical matter of engineering. He does not, as far as I know, use the word ‘physical’. I think it is probably a misconception to think that Descartes would have regarded souls as ‘non-physical’.
3. Past and future form a further duality for physics. The present is an interface between ‘what has happened’ and ‘what is happening’, the descriptions of which take quite different forms. For Whitehead this interface was an ‘occasion’ in which the present takes up or ‘prehends’ the past with the present being the ‘mental pole’ and the past the ‘physical pole’ of the occasion. In modern physics this duality seems to be represented in Schrodinger’s equation by the distinction between the progressing function Psi and the field of potentials V that Psi progresses through.
There are all sorts of other ways of expressing what seems to be a basic duality in any form of physics and which corresponds broadly to what Bohr called complementarity. Both Bohr and Wolfgang Pauli hoped to show that complementarity was a wider principle than just the incompatibility of certain technical descriptions like position and momentum. I think they were right in this and have expanded the idea in the online essay Reality, Meaning and Knowledge.