Descartes’s metaphysics is traditionally seen as more ‘dualist’ than contemporary physicalism. The grounds for this seem shaky, however.

Reading definitions of substance dualism on major web sources, there is a confusion between the modern concept of ‘substance’ as ‘type of stuff’ and the Cartesian concept of ‘substance’ as individual entity, not dependent for its existence on other things (such as parts).

If we employ Descartes’s usage then strictly speaking he only defines spiritual substances – including God and souls. Matter is infinitely divisible so any material object would not be a true substance in his framework. It remains true that he is describing two different patterns of dynamic disposition – one characterizing spirit and one matter, but ‘substance dualism’ seems a poor fit for what he is saying.

If we employ the modern usage of substance then it is hard to classify souls as belonging to a type of stuff since they lack the properties of ‘stuff’. Moreover, modern science identifies all sorts of types of substances. So substance dualism in this sense is hardly outside physics. And where authors stipulate they do not refer to this usage anyway, it seems to slip in unintentionally.

My impression is that to understand Descartes it is necessary to consider what physics was like at this time. Descartes only uses the word ‘physics’ once in the Meditations, when discussing legitimate ways of explaining the nature of God. Although he does not say so explicitly, he implies that ‘God’ is a central part of his physics, since all matter is inert and incapable of generating motion, so can only move with the constant action of God. The argument for God’s existence is about the regularity and intelligibility of the dynamic world around us, i.e. physics, not the presence of anything outside physics. God is in a sense his word for the reason why the laws of physics hold – something that all modern physicists would agree must exist.

The features of spirit, in the form of God and souls, are thus the ability to initiate motion. Spiritual substances are also unextended – in the sense of not excluding anything from a space – and indivisible or simple. They are also capable of logical steps or reason, which would seem to involve the generation of an outcome from an input in a non-mechanical way. Spiritual dynamics in this sense are a very basic part of modern physics, in the form of forces, in everyday life electromagnetic forces, which were unknown to Descartes. In simple terms Descartes’s spirit is now known as force, although precise roles played have changed from his primitive hypotheses.

As Jean-Pierre Changeux pointed out recently, Descartes is really very ‘physicalist’, being as mechanistic as he can. He tries to give more precise local explanations for our thoughts than even modern neuroscientists do. He is probably more physicalist than they are.

Is it about time that Cartesian Dualism was exposed for being the myth it is?