For a long time now, I've been a little suspicious of the tidy distinction philosophers make betweeen the physical and the non-physical, or between the material and the spiritual, or the body and the mind (or soul). I still consider myself a substance dualist because I think there is definitely a distinction. What I'm suspicious of is that the distinction is as tidy as philosophers have made it out to be, especially since Descartes.
The whole mind/body problem was born out of the way Decartes distinguished the mind and the body. He said the body was an extended non-thinking thing, and the mind was a non-existended thinking thing. "Extension," I think, was Descartes' way of referring to anything that's part of the physical material world. Since then, dualists have kind of equated the mind with the soul and the spiritual in general. But surely, there must be more to the spiritual than mere first person subjectivity.
Anyway, this tidy distinction in dualism gave rise to the interaction problem. If the physical world and the mental or spiritual world have no properties in common, then how do they interact? This problem can be formulated in a number of ways, but I'm not going to go into that.
This morning, I was thinking about a video I saw on YouTube yesterday on the PBS SpaceTime channel. Matt was talking about the difficulty of detecting neutrinos and how many neutrinos there are. I was thinking this morning about how some physical objects are easier to detect than others. A table is easier for us to detect than a magnetic field. Neutrinos are nearly impossible to detect because they hardly interact with anything. There are supposedly trillions of neutrinos traveling through your body every second, and nary a one is interacting with you. In the video, Matt said if you shot a beam of neutrinos through a wall of lead one light year thick, only half of the neutrinos would be attenuated.
Neutrios aren't the only things that are hard to detect. There's also dark matter. Dark matter doesn't interact with light. The only reason we know it's there is because of its gravitational effects on large scales. We can tell from the way galaxies rotate that they are surrounded by a cloud of dark matter. Some have speculated that maybe dark matter doesn't exist, and the solution lies in updating our current theory of gravity (general relativity). From what I gather, most physicists seem pretty sure that dark matter is composed of some kind of exotic particles, though, so they don't think the problem is with our theory of gravity.
Then there's dark energy, which we only know about because the expansion of the universe is accelerating for no obvious reason. We call its cause "dark energy," which is a way of saying I-Know-Not-What.
So the physical universe is full of surprises. There's a whole range of detectibility, which makes me think this range must extend beyond the detectable. If there's something as elusive as a neutrino (which is part of the standard model), then maybe there are other things slightly more elusive. They are just beyond the horizon of what we, made of standard model particles, can detect or interact with. The standard model doesn't account for everything.
To me, this kind of blurrs the distinction bewteen the physical and the non-physical. Suppose there are two particles, A and B, that cannot interact with each other. But suppose there's third particle, C, than can interact with both. So there is a way for A and B to interact with each other indirectly through C. This sort of thing may tie all physical things together even if some can't interact with others.
Suppose C didn't exist, though, or was at least extremely rare. What if there were people made of A-type particles, and other beings made of B-type particles. If each considered themselves "physical," wouldn't they consider the other "non-physical," or "non-existent" if they knew about each other at all?
Imagine there being physical things that don't interact with anything else at all, not even indirectly. This might raise questions about what it even means to exists. J.P. Moreland once said that to exists is to have properties. But what sorts of properties would something have that couldn't interact with anything? It's hard to imagine. But lack of imagination isn't a good reason to say it doesn't exist. Maybe we could say that for all practical purposes it doesn't exist since it plays no roll in the development of the universe.
But anyway, if there are these physical things that don't interact or that interact weakly or with very few other things, then why call them physical at all? What does it mean to be physical?
I remember when I was a kid being told that matter is anything that has mass and takes up space. Shortly after that, I was told there was this distinction between matter and energy. Light, I was told, was a form of energy since it doesn't have mass. But as you delve into the physics, you learn that a photon is just another particle in the standard model, and there is no tidy distinction between mattere and energy. In fact, they can be converted to and from each other. Some even say energy is just a number and isn't a substance at all.
I learned later on that even space and time itself are "stuff." They are physical. They can bend and be manipulated. The more we learn through physics, the more we have to tweak what it means to be physical in order to accomodate what we know to be out there. If there were more out there, like particles that don't interact, and we were somehow able to discover them (how is a mystery), then I suppose we'd have to tweak the meaning of physical even further.
So if it turns out there are these things called spirits, souls, disembodied minds, etc. that populate the spiritual world and that can, on occasions and in very weak ways, interact with the physical world, then wouldn't we just tweak the meaning of physical even further to accomodate them?
Just as there are physical things that have properties we may not know about, maybe the spiritual world does, too. And maybe it's in the murky areas in the fringes of their detectability or knowability that they share properties with physical things and can interact with each other. Of course if they can interact, then they are at least in principle detectable. But they may not be detectable in practice.
When the mind interacts in the brain, the interaction could be very subtle. One small interaction at the subatomic level could cause an avalance of causes in the brain. The avalanche can be detected, but the initial cause is lost in a sea of apparent quantum randomness. Finding it might be like finding Waldo or even worse.
But I am skeptical of the notion that spirits or disembodied souls don't have any physical properties at all--that there's a tidy distinction between them and everything we know to be physical. I suspect, but am far from certain, that spirits can have location, for example. If they can be located within spacetime, then they probably have a size as well. Contra Descartes, they can have extension. If they have location, extension, and size, then perhaps they can interact with spacetime itself. After all, if something has spacial properties, and you can bend spacetime, then this would surely affect that thing. By interacting with spacetime, a spirit can interact indirectly with quantum fields, and therefore through the particles that emerge in quantum fields, which would include all the standard model particles. Maybe spacetime is the "C" that bridges the causal gap between the spiritual and the rest of the physical world.
I don't know, though. Maybe what we think of as location when it comes to spirits, angels, etc., has more to do with where in the physical world they interact rather than where they are themselves. We think my spirit is in my body and yours is in yours because that's where they interact, not because they are physically located within us.
This is all speculation, of course. This is the sort of thing I think about when taking a shower or going for a walk. I suspect, though I cannot argue, that if we could exhaustively know all the properties that are had by all things that exists that there'd be this range--this spectrum--in which we go from the hard physical world to the non-physical world, and somewhere in that range, there would be a murky area in which is isn't clear whether we are talking about something that's physical or non-physical.
Don't get the wrong idea, though. I am not an idealist, a physicalist, or a pan-psychist. I am still very much a substance dualist. Just as there is a distinction between a pile of dirt and a hill, or baldness and having hair, in spite of there being a gray area, so also do I think there is a distinction between the physical and the spiritual, even if there is a gray area.
