This is the first of a series of three blog posts about my book. It will introduce the main idea behind it, and highlight a few applications. The second post will present one of the chapters in a bit more detail. Finally, the last post will try to identify some future venues of research where the hypothesis of the book might prove useful. I will keep the posts without too much detail, so as to induce some curiosity in the reader to pick and read the book itself. The posts will appear here in the next 15 days, and I will try my best to get involved in the discussions, should there be any.

The basic hypothesis of the book is extremely simple. It is that we could take mental states to be realized by, or identical to (I myself assume identity in the book, but nothing hinges on this choice) CNS + PNS neural subsystems. Is this idea new? I provide several quotations from the classical philosophy of mind literature that show how philosophers assume that mental states, both sensory and non-sensory, are realized by brain states. Even the embodied and the extended approaches to the mind seem to neglect such a philosophical hypothesis when it comes to sensory states, such as pain, and some authors who do stress the bodily aspects of non-sensory states assume a body-in-the-brain type of approach, which still has the brain as the central mover, and the body merely as a contributor to a body image in the brain.

Which CNS+ PNS subsystem realizes which type of mental state is an empirical question.I discuss the state of pain, and stress that it is best to be thought of as a state that involves large areas of the brain, a gate system in the spinal cord, as well as various PNS components, depending on what the source stimulus is. Other types of mental states will have other such subsystems as their realizers, and it is a question of empirical research into such correlations to find out which. What is distinctively philosophical here is the conceptual point that there is no a priori or a posteriori reason, for what we know so far, not to postulate that the peripheral components involved in virtually all mental states are not merely causal contributors to the occurrence of those states, but constitutive of them.

For instance, in the standard case of pain, that is, when pain is caused by injury of some bodily tissue, I say that the activity of C fibers A-delta (which are fast conducting, myelinated bundles, responsible for the first, sharp stage of pain) and of A-delta C fibers (slow conducting, unmyelinated bundles, responsible for the second, longer and dull type of pain feeling) is constitutive part of the realizer of the quale of pain. Most philosophers seem to assume that the quale of pain is realized by some region of the brain, and the PNS components, as well as the non-brain-based CNS components (in this case the gate system in the spinal cord) are merely causes of the state of pain. I don’t have an argument for my view, but they don’t have one either for their assumption. So we are doomed to beg the question against one another. The difference is that I beg it with more philosophical benefit, if what I say in the book is more or less right and credible.

So here are some highlights of where the Peripheral Mind Hypothesis proves to have some philosophical payoff, according to what I claim in the book.

Zombies. This is the part I’ve become least sure about, after the experience of not having convinced any of the philosophers who read the manuscript in draft form. However, I still endorse what I say (I’m the author, after all, and the book is fresh 🙂 ). What I try to show is that the zombie intuition can be weakened once we include the PNS as a constitutive component of the neural basis of conscious states. I reject the need to appeal to phenomenal concepts, as well as any physicalism that is based on denying the conceivability-possibility entailment. The first step is to stop thinking in terms of persons as subjects of experience, and think rather in terms of nervous systems. This shouldn’t bother anyone, except substance dualists. Property dualists should be happy with it since they are substance physicalists. And what else is the physical subject of experience than a nervous system? The next step is to ask: what does it mean for a nervous system to be conscious in the actual world? It means to be activated in some ways. The zombie intuition is that you can conceive of a copy of this conscious nervous system such that the copy lacks consciousness. What happens if the property of being conscious is partly realized by the PNS (the model I use in the book for this view is that of distributional properties, first discussed by Josh Parsons — conscious state types are then realized by ways to distribute electrical activity over parts of the nervous system)? I claim that at the level of an individual nerve fiber we have a specific conscious property, the fibers own constitutive contribution to the global state of consciousness. Finally, we ask the question “what does it mean for the fiber to be conscious?”

The basic point is that if we focus on the consciousness specific to an individual peripheral nervous fiber, the intuition is that what it means, not just actually but simpliciter, for that fiber to be conscious is simply for it to be active. At that level of simplicity, at which we got rid of the temptation to involve anything like a person in the thought experiment, to be conscious is for the fiber to fire. This has the consequence that a zombie fiber (a zombie foot, a zombie retina, etc.) is inconceivable. But since we can replicate this all the way up to the higher levels of the nervous system, a zombie brain is equally inconceivable.

The mad pain problem. Mad pain (i.e. a pain quale caused by, say, moderate exercise on an empty stomach, and causing the desire to solve mathematical problems) is logically impossible, because part of the realizer of the pain quale are the nociceptive peripheral fibers. The fibers involved in moderate exercise don’t deserve the name “nociceptive fibers”, hence the alleged quale above does not deserve the name “pain quale”.

The problem of pseudonormal vision. Pseudonormal vision is an alleged actual version of spectrum inversion. There are two types of partial color blindness: (A) when the green photopigment is contained in both the G and the R cones, and (B) when the red photopigment is contained in both the G and R cones. The genes responsible for case (A) and (B) can in rare cases be present in one and the same person, thus such persons, called ‘pseudo-normal’, although not visually defective in terms of normal color discriminations, have their photopigments swapped between the G and R cones. Hence, according to Martine Nida-Rümelin (“Pseudonormal vision: An actual case of qualia inversion?”, Philosophical Studies 82 (2):145-57, 1996) they are actual cases of spectrum inversion. My response is that the above argument depends on assuming that qualia are realized by brain states; once we include cones and photopigments as constitutive of experience, the possibility of inversion vanishes. It is in virtue of the photopigments that that one’s experience has the color quale that it has, hence, me and my alleged pseudonormal twin will experience the same colors, because the same photopigment is activated when we look at the same surface. A photoreceptive cell deserves the name ‘G-cone’ in virtue of containing the G pigment; the morphology of the cones has no relevance, as proven by the history of neuroscience (see the book for details).

The China-brain problem. I think that why most people got moved by Ned Block’s China-brain thought experiment is because they focused their attention on the wrong side of the China system, namely on the brain. Suppose the government of China is a bit less ambitious and is content with only realizing a brief pain sensation via a system of people and radio transmitters. What happens is that I offer my nociceptive nerves and the corresponding motor nerves to be used by China for the experiment. They will put me to sleep, disconnect my pain-related PNS components from my brain and connect them to the China-pain CNS system. I will wake up, suppose, with all the other cognitive components intact, except for those involved in the sensation of pain. Intuitively, I will be zombified, pain-wise: I won’t feel pain; yet, when my skin is hurt, my relevant muscles will contract and I will avoid the stimulus, I will scream, and so on.

What about the CNS states in the China-pain system? If in Block’s experiment you intuited that the China-brain system is not conscious, you will intuit here that the China-pain system is not in pain. I also intuit that in this case I am not in pain and the China-pain system is not in pain either. Yet, I do not intuit that there is no pain at all instantiated by the composite system of my body (PNS) + China-pain (CNS). The only puzzle about the original China-brain system is a puzzle about who is conscious, not about whether there is any consciousness somewhere in the global system.

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There are several other puzzles that the PMH offers novel solutions to: the problem of triviality for machine functionalism, the possibility of a brain in a vat, the problem of wide mental states posing a threat to physicalist supervenience, as well as the problem of how to distinguish between causal and constitutive contributions of nervous processes to experience. I will present the last one of these next week in a new blog post, and leave the other three for the readers to check out in the book.

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