“It is sometimes objected that physical and mental states could not interact since there is no causal nexus between them. However, one lesson from Hume and from modern science is that the same goes for any fundamental causal interactions, including those found in physics. Newtonian science reveals no causal nexus by which gravitation works, for example; rather, the relevant laws are simply fundamental. The same goes for basic laws of physical theories, and the same presumably applies to fundamental psychophysical laws. There is no need for a causal nexus distinct from the physical and mental properties themselves.”
– David Chalmers, The Character of Consciousness
An interesting statement, and one which is rather untroubled for a person who wishes to subsequently make an argument for property dualism. However, I think most advocates for substance dualism would be troubled by the implied requirement in the above statement, which is conformity to the explanatory requirements of causation. The desire for most folks with a stake in substance dualism is for a substance which is truly separate – from the requirements of location and dependent identity associated with being physical – and not merely irreducible. I think there are profound problems with the irreducibility of the mental and ‘downward causation’. However, it seems like there’s a problem with the atomic characterization of a distinct “mental substance” in and of itself.
To get an idea of the role which a substance fulfills, it is helpful to examine a relatively well defined and uncontroversial representative of physical substance: the electron, for example. The electron is describable entirely on the basis of its properties. It has a negative charge, mass, spin, etc. In a sense, it is just a receptacle for its properties. But the properties of the electron do not entirely suffice to explain the electron and why we need it. We could account for all of our experience of those properties without a particle for their residence. We could use a bundle theory, like Hume proposed for the mind.
We can simply speak in terms of functional conditions, in other words, the bare circumstances under which the properties are manifest. For charge, we can say the same thing that we say about magnets – a repulsive or attractive force occurs when a certain orientation of physical objects occurs. Micro structural explanations be damned, that just is magnetism. Likewise, we can say that elements accrue certain, conventional, mass units as they accrue charge units. We don’t need to refer to a particle to make this functional explanation.
Instead, the particle serves a historical role. We experience mass and charge properties necessarily at a certain place and time, not just under certain conditions. We know where to look to find a certain mass and charge, even if we do not perceive them at the moment. The particle has an inert state in which we know of it merely by historical reputation – it is there because previous circumstances demand it.
Is a mind like that? Does a mind have an inert or ground state where it is, just because previous circumstances demand it? I don’t see how. Minds are elicited. Mental occurrences are toward, of, or about something. Our subjectivity and motive pertain to their immediate circumstances. Even if previous circumstances condition it (I think critically), previous circumstances don’t demand that red looks like it does to me. There’s no need for an atomic mind to explain mental “properties”. In fact, those properties defy attribution. That’s the upshot of the knowledge argument. The argument militates against a mental type, and a third person ontology of the mental, generally.
Advocates of a mental substance can equivocate about the possibilities of the stuff. Maybe it isn’t bound by the requirements of normal causal relations, which demand relative location and defining interactions at least. But then, what is it? I think the substance dualist is faced with providing an account of pure mentality – the inert mind, mind without content, mind without active reference. I can provide such an account for inert electrons. I can simply refer to locality and the demands of previous circumstances; in other words, the notion of causality concomitant with our experience. Without recourse to those explanations, a condition which true separation of substances seems to dictate, I don’t see how such an account is possible for mental substance.