I was thinking about Substance dualism recently, given a conversation that I was having in which a friend identified me as (perhaps even ‘accused’ me of being) a ‘substance dualist’. Although I accepted the label, I also immediately cushioned myself against being characterized as a Cartesian with respect to the Soul by noting that I did not accept that the soul and the body were two different substances. My view is technically not one of substance-dualism, but rather one of hylomorphism (the standard view among Medieval theologians). According to the hylomorphic view, the substance of the ‘human being’ is comprised of form and matter, such that the soul is the form of the human being, while the body is the matter. Thus, the substance is ‘human-being’, the form is the ‘rational soul’, and the matter is the ‘body’. On this view of the human person there is no difficulty of interactionism, a difficulty which invited many innovative systems of philosophy in the early modern period.
After having briefly explained this view, it occurred to me that if one can speak of the soul apart from the body as an individual substance (thing) of some kind (such that we can speak about the saints in heaven as persons even if they aren’t properly ‘human’, at least until the resurrection), then we must explain how we can speak of the soul as an individual substance and also the form of a substance. It seems to me that we should be inclined to think something like Leibniz’ proposal is true: namely, that matter supervenes on substance. In other words, the material world is a result of the immaterial substances having relations of perception.
Problems with this view might exist. For instance, I wonder how to make sense of the commitment to Sensitive or Vegetative souls being entirely material souls (rather than immaterial), but it might be possible to make sense of that given an analysis of what we (can) mean by ‘material soul’. Moreover it isn’t as though we are bound to that Thomistic articulation.
In any case, it was just a thought.