Here I will be arguing that the thesis that there is some non-physical stuff in addition to the physical stuff of the universe that explains the existence and nature of consciousness (and possibly other things as well) doesn’t actually explains what it sets out to explain, and thus is a bad philosophical theory. Well, actually that is a bit of an exaggeration. Really what I am showing is that it can’t explain why we believe (intuitively) that such stuff exists, independently of how well that stuff happens to explain consciousness (and whatever else it is used to explain). Even given that it could still turn out to be such a wonderful explanation of consciousness that we accept it anyways, as the best explanation we have, despite the lack of direct evidence for it. However theories that invoke non-physical stuff to explain consciousness generally do a terrible job at explaining the nature of consciousness itself (questions such as “why is my experience at this particular moment the way that it is?” are never really answered, nor is any method proposed by which they could be answered; the nature of our conscious experiences are treated simply as brute facts). Thus the acceptance of such theories (by those who accept them) is motivated primarily because they explain the intuitive way we think about consciousness. So, provided that we can come up with some theory that explains consciousness without appealing to a non-physical world, we would be thus motivated to reject theories invoking non-physical stuff as providing inferior explanations.*
To start with I must introduce an introductory claim and an introductory definition. The claim I will lean on is the existence of psychophysical parallelism, which says that for any particular physical state of the system there is only one possible associated mental state, even if the mind really is a separate non-physical thing that does not have a causal influence on the physical world. There is not really an argument for this claim, but rather an overwhelming amount of evidence, from fMRIs to brain damage, which points to the existence of such psychophysical parallelism. Which is not to say that questioning the existence of such parallelism is impossible, but rather that the burden of proof is on those who question it to explain why all the evidence points so convincingly to the existence of such a parallelism and to provide some evidence pointing to the absence of such a parallelism.
My introductory definition is much simpler, and probably more contentious. But it is a definition, and so to question it is really to insist that the words in question have a different meaning. With that said the definitional claim is that the physical (observable**) world is causally closed. Of course I have no intent of getting into why exactly this is true by definition, but I will point out that it should seem natural enough when you consider the possibility of something non-physical (non observable) having a causal influence of the physical world. If that were the case we could notice its effects, and then through them observe it, making it part of the physical world after all.
Now consider a theory that proposes that non-physical feature of the world X is responsible for some feature of consciousness. And let us further suppose that Y is some state of the non-physical world that excludes the possibility of X. Now the story involving how we know about he non-physical world is supposed to go something like this: X is in the past. We then form a thought to the effect that “X is the case”, and at the same time our physical brains enter a state corresponding to “X is the case”. But this is insufficient to know that X is the case, as is demonstrated by the following possibility: suppose Y was the case, but the physical world was exactly the same (since the non-physical doesn’t have a causal influence on the physical there is nothing preventing this). Since the physical world is the same our brains still come to be in the state corresponding to “X is the case” and thus by psychophysical parallelism we must be thinking that “X is the case”. Since in both situations we believe that X is the case the fact that we believe that X is the case cannot be evidence for X really being the case. Or, in other words, we can’t know that X is the case. Now some might question this, arguing that the thought that “X is the case” can’t occur without X occurring in the past, and so that this counterfactual setup is impossible. But that is begging the question, because what is under debate here is whether we can know things such as the rules of the non-physical world. Thus we can’t appeal to knowledge about how the non-physical world works in order to justify the claim that we have knowledge about the non-physical world.
This demolishes the ability of theories involving a non-physical world to explain our intuitions and claims about that world. Our intuitions and claims about that world are caused completely by the physical world, and thus can’t give us any information about it, and can be explained without appeal to the non-physical world. This means that even if we somehow established that a non-physical world must exist we would have no way of connecting its features to things like consciousness and the nature of experience, because making those connections would require the ability to have evidence that the non-physical world was actually in a particular state, and then using that knowledge to correlate events in the non-physical world with conscious experience. (Of course there is the hidden premise here that we can’t know anything a priori about the non-physical world, but that is obvious, I would hope, since it looks like it is impossible to know anything a priori.)
* Why the long introduction? Well, I have been thinking a lot recently about the nature of philosophical claims, why we believe them, and how we should decide whether to accept or reject them. So I have turned my argument here into a bit of a test case, showing that we can do philosophy within the model of how philosophy should be done that I am experimenting with.
** Observable here meaning something like: in principle we can have reliable information about it such that it is available to a scientific/experimental study. And naturally what is physical is best defined as that which is in the domain of science.