Could we have non-physical souls that control our bodies by causing effects in the brain that propagate via the nervous system to our muscles? Contemporary naturalist philosophers—philosophers who understand the world solely in terms of physical causes—have three main objections to this idea.
The first objection is the worry that this interaction would contradict what neuroscience has to say about our actions originating in the brain. I won’t focus on this here, but I can say that neuroscience does not have a sufficiently detailed understanding of our decision-making to rule out the possibility of a non-physical component.
The second objection is known as the “interaction problem”: How could a non-physical cause have an effect on something physical? The interaction problem is a red herring (a “red herring” is a fallacy in argumentation that distracts from the real issue). Yes, it is very mysterious how a non-physical cause could have a physical effect. But (at least given a non-deterministic physics like quantum mechanics) it is no more mysterious than how any cause could have an effect. Sir Isaac Newton said that the sun has a gravitational effect on distant planets. Philosophers wondered: How could that be? Certainly such causation at a distance is mysterious, but it is not just a thing’s being at a distance that makes it mysterious—it is causation that is a mysterious tie between distinct events. How can one thing cause another? That is the mystery of causation, but it is no more puzzling when one of the things is physical and the other is non-physical than when they are both physical.
Maybe it is puzzling how a non-physical cause can have an effect on something physical, because the non-physical and the physical are “very different” from each other. But non-living unconscious things and living conscious beings are very different from each other, and yet non-living unconscious things constantly affect living conscious beings—oxygen molecules, for instance, constantly keep us alive. Science is full of things that are “very different” interacting causally with one another. Indeed, naturalists think that non-living matter can cause life, and non-conscious matter can cause consciousness.
The third objection is based on the Closure Principle: The physical world is explanatorily closed—the explanations of physical states of affairs are always physical in nature. The closure principle would let one be confident that scientists don’t need to look for miracles—that explanations of physical events will never go beyond science. If the Closure Principle is true, then no physical event in the brain has an explanation by means of a soul.
I’m going to argue that the Closure Principle is more likely to be false than true.
My argument has two parts. The first part will be to establish the No Physical Explanation Thesis:
No Physical Explanation Thesis: Whether or not the Closure Principle is true, some physical states of affairs have no physical explanations.
The second part will argue that if the above thesis is true, then the Closure Principle is more likely to be false than true.
So, first, I need to argue that some physical states of affairs have no physical explanations. A quick argument for this thesis is to consider all of physical reality as a whole. This constitutes a gigantic physical state of affairs. This gigantic physical state of affairs cannot have a physical explanation. For if it had a physical explanation, that physical explanation would be a part of the gigantic physical state of affairs, since that gigantic state contained all of physical reality. But then the explanation, in explaining that gigantic state, would also explain itself, since it itself is a part of the gigantic state. But no physical state of affairs can explain itself—that would be like the absurdity of being one’s own parent.
A second (related) argument is that if every physical state of affairs has a physical explanation, we end up with a vicious infinite regress of physical states of affairs. Physical state #1 is explained by physical state #2 (maybe, my existence is explained by my parents’ existence), which then must be explained by physical state #3, and so on. So we have an infinite regress of physical states of affairs. But the regress does not stop there. For the whole infinite regress will be a physical state of affairs, and hence will need an explanation. So we will have another regress, and another, and so on. This is really absurd.
A third argument is that current science gives us good reason to think that physical reality is not infinitely old, that it came into existence some finite amount of time ago, maybe in a Big Bang. If so, then the state of affairs of physical reality coming into existence won’t have a physical explanation.
These three arguments for the No Physical Explanation Thesis are basically adaptations of the Cosmological Argument for the existence of God, but I am not concluding from them that there is a God—just that something physical has no physical explanation.
So now we’ve learned that some physical state of affairs has no physical explanation.
Let’s give a name to some such physical state of affairs—call it “Blob.” Blob, then, has no physical explanation. There are now two possibilities. Either (1) Blob has no explanation at all, or (2) Blob has a non-physical explanation.
If Blob has a non-physical explanation, then the Closure Principle is false, since the Closure Principle said that no physical state of affairs has a non-physical explanation, and here is Blob—a physical state of affairs with a non-physical explanation.
Of the two options, which is more likely? Blob having no explanation at all? Or Blob having a non-physical explanation? I will argue that think the likelier option is the latter—that Blob has a non-physical explanation.
For physical states of affairs are contingent—they don’t have to be there. Blob, thus, doesn’t have to be there. The idea of something contingent having no explanation is like the idea of something coming into existence out of nothing. It is intuitively absurd. Much more absurd than the alternative, namely that it has a non-physical explanation. Maybe it’s hard for something physical to happen because of a non-physical cause—but it should be far harder for something physical to happen for no cause at all. If we think that that something contingent could happen for no cause at all, it should be even easier for it to happen due to a non-physical cause. So it is more likely that Blob has a non-physical explanation than that it has no explanation at all. But if it has a non-physical explanation, then the Closure Principle is false. So, most likely, the Closure Principle is false.
Moreover, if we admitted that a physical state of affairs, namely Blob, could take place for no reason at all, with no explanation, then it would be a wonder why all sorts of other physical things don’t happen for no reason at all. Why doesn’t a dog-headed person suddenly pop into existence for no cause at all in front of me? Why doesn’t a golden mountain materialize in the middle of the Pacific Ocean out of nowhere? I can give a very simple explanation of why all these infinitely many strange things don’t happen: all physical states of affairs have to have explanations. But this explanation also implies that the Closure Principle is false. For we know that Blob has no physical explanation, and so Blob’s explanation must be non-physical.
The Closure Principle was introduced in large part to save science from giving up on physical explanations and looking for supernatural ones. But if we accept the Closure Principle, then we have to say that some physical states of affairs have no explanation at all. And it is much worse to be willing to give up completely on a search for explanation than to be willing to give up on a search for physical explanation.
We should thus think that most likely the Closure Principle is false, and hence this argument against interaction between the soul and the body fails as much as the previous ones did.