Is a purely physical world possible? Our world contains mental attributes, but suppose we strip them completely away, so that nothing mental remains: will the world that is left be purely physical? I don’t mean will it actually lack all mental attributes—by hypothesis it will; I mean will it lack even the potential for mentality. Will it be nomologically impossible for mental attributes to exist in this denuded world? Some people believe that our world contains hidden mental attributes, down to the level of atoms—our friends the panpsychists. Suppose we strip these away too—are we then left with a zombie world without even the potential for consciousness? If we think of the hidden psychic attributes as fundamental independent properties of matter, like gravity and electricity, then it should be possible to imagine them away, leaving nothing even potentially mental in the world. We preserve just ordinary material entities and the basic physical forces (gravity, electromagnetism, the weak force, and the strong force), with no mental dimension at all: have we not created a zombie world in which even the potential for mentality has been eliminated?
According to traditional forms of dualism, we should have. We have removed any Cartesian mental substances, any irreducible mental properties, and any proto-mental properties of the panpsychist type. This should then be a world in which minds cannot arise (without infusing it afresh from the outside)—a permanent and principled zombie world. The raw materials for the emergence of mind are just not present, so the world will remain without mind forever and necessarily. Thus a zombie world is conceivable—a world without even the possibility of mind. Our world is not a zombie world, since evidently it can produce mentality, but we can modify it in such a way as to remove mentality from the picture altogether—or so it might be thought.
Yet it seems to me that it is really not clear that such a thing is possible. It is not clear to me that a world with only the usual particles, planets, and stars, with the usual physical forces, could not produce a world with mentality (indeed, I think our world is just such a world). In a world like this complex life could arise, and this life could involve brains like ours—physical duplicates of our brains. So prima facie the potential for mind exists in a world meeting the description given. Granted, we have no idea how this might come about, but ignorance is not the same as logical or nomological impossibility. What we have to recognize is that we just don’t know enough about the nature of the basic entities and forces to rule out the possibility of mind arising from them. This is not because there is some additional mental ingredient to matter—an independent mental dimension to things—as panpsychism supposes. For we have removed by stipulation all such mental ingredients in our thought experiment. It is because the basic physical entities and forces operating in the world might themselves contain the potential to produce mind (as they contain, surprisingly, the potential to produce life). We cannot rule this out a priori, and it seems to be the way things have to be. That is, it might be that a purely physical universe is never “purely physical”—it always contains the potential for mind. Just as there could not be zombie brains—mindless physical duplicates of our brains—so there could not be zombie universes—physical universes incapable of producing minds. Maybe any universe containing gravity and electromagnetism will have the resources to generate mind. That is what appears to be the case about our universe, putting aside dualist theories—even though we admittedly cannot make sense of it. The idea of a zombie universe is thus a fantasy, a trick of the imagination. A physical universe is always (potentially) a mental universe.
How much of our physical universe can we strip away and still leave the possibility of mind? Now that is an interesting question, and a difficult one to answer. What if we removed electricity and magnetism, leaving only gravity (plus the strong and weak force)? Gravity would still give us motion, but would it give us mind? Is motion somehow sufficient for mind (behaviorists would say so)? Or what if we abolished gravity and left only electromagnetism? We would still have motion, and brains could still be powered by electricity, which seems to have something to do with mental processes. Maybe gravity is not necessary for mentality but electricity is. But what if we eliminated both—what if we got rid of forces altogether? Assuming such a thing to be coherent, we would have left only bits of matter with various modes of extension—with no motion and no change of any kind. Would mind still be possible under these austere conditions? That does seem inconceivable—so forces of some sort appear necessary for mind to exist. There really could be a mindless zombie world if forces and motion were removed.
However, it is far from clear that a world with the same possibilities of motion as our world could be a genuine zombie world. Maybe motion itself contains the resources to produce mind, if only we understood it better. But then there must be more to motion than meets the eye (or the human intellect). Past thinkers found motion exceedingly perplexing, even supposing that it called for divine intervention; maybe they were onto something. We have a highly etiolated and abstract conception of motion in current physics, and hence of the forces that produce it, but it is possible that it possesses a much richer nature than we imagine. In fact, it is difficult to make sense of mentality in a physical universe such as ours without some such supposition. We should be agnostic about what aspects of physical reality are irrelevant to the production of mind. A lot more might be relevant than we tend, in our ignorance, to suppose. Can we really rule out any aspects of physical reality as contributing to the existence of mind? Mass, motion, geometry, space, charge, gravity, and atomicity—all might be necessary preconditions of mind. If so, removing mentality from the world would be removing everything physical from it. What we call “the physical” might be much closer to what we call “the mental” than we tend to suppose. 
 Need I add that these thoughts are at the very edge of human knowledge and human intelligibility? To call them speculative is understating it.
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