Nikki asked:

The mind-body problem states that metaphysical entities cannot interact with physical entities. I don't
understand the problem because gravity interacts with all physical entities and gravity is not physical.
Or is it? The same with magnetism. These are non physical substances.

and Katie asked:

Is it impossible for a nonphysical thing to interact with a physical thing?

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Now this is an intriguing question that has long fascinated me. What do we mean by physical? I
asked the noted central-state materialist David Armstrong this very question a few years ago, and he
said that is it anything that appears in Physics. So, I said, if souls appear in some future physics
theory, then would souls be physical? Yes, they would. [Note: I'll use 'soul' here free of any religious
connotation — you may substitute 'thoughts' in most cases without major damage to the account.]

I think that there is often a confusion between the material and the physical. Material things are often
thought about in billiard ball terms — atoms and the void, as Democritus would put it. Physical entities
have long been thought to go beyond this, to include such things as gravity and magnetism — force
fields.

Isaac Newton himself did not like the idea of action-at-a-distance, a key attribute of his gravitational
theory. He thought things ought to touch to interact — that the physical should be material. We are
much more comfortable with forces as physics, and it takes a rare mind like Nikki's to see the
emperor's clothing here.

Indeed, some modern physicists who dislike action-at-a-distance claim that all field interactions must
be due to the constant swapping of yet more hard-to-put-your-finger-on particles between those
particles which are interacting (so they invent gluons and gravitons and other things that, as far as I
am aware, nobody has ever seen). This may be plausible for nuclear forces, but it seems
extraordinarily far-fetched to me for gravitational interactions, which happen between all the masses
in the universe.

So when we say that metaphysical entities cannot interact with physical entities, I think we are saying
that there is no known way for things like the soul, or thoughts, to enter into the well known chain of
physical causes and effects (including gravitational and magnetic causes). Descartes (writing,
remember, at about the time of Newton's birth) did think of all matter/physical substance as having
mass, extension and location. For him, with a billiard ball model, how something lacking mass and
extension could affect anything that did have these properties was a problem.

Fields have extension but no mass. They are not matter, but they seem to be physical. They are still
mysterious, though, in that their nature is mysterious. Their effects, on the other hand, are well
understood. [I would, though, hesitate to call them substances — substance talk is not very
fashionable now.]

So, if we grant fields affect matter, where does that leave us? Are souls like fields? Do souls have to
lack extension or location as Descartes claimed (if they do, it makes it hard to explain why my soul is
mine, not yours)? Is it true that they can't affect the material, as fields do? Roger Penrose has a
theory that thoughts do affect the collapse of quantum waves within the brain in such a way as to
affect the material world. There is a lot more to Nikki and Katie's questions than introductory
philosophy books let on.

[Disclaimer: none of the above means that I am a dualist. I go more for the double-aspect theory,
though I will freely admit that mind-body is a difficult problem. A double-aspect theory claims that a
physical description is only describing a part of the totality of that reality, and that a mental description
can capture another aspect of the same reality.]

Tim Sprod