I have found myself more than once in conversations with people who think that in order to explain consciousness and the human mind you need to resort to something "beyond the physical world", something that transcends all the experiments and research done on the mind since it started to be studied in a scientific way. Despite my attempts to offer explanations, they insist in the claim that something like that can't be measured, and that they have seen, felt, or heard from sincere people, things that science can not explain. Leaving anecdotal evidence aside, we should ask ourselves whether the existence of a metaphysical mind as an entity separate from the body, or what many religions call soul or spirit, is something that can really exist.
It could be said that what we do with our mind is to think and take decisions, among other things. With it we can decide to move an arm or a leg, for example. Or maybe start running, or solve an equation with pencil and paper, or even write a blog post. All the actions I mentioned would be impeded if the nerves that connect the brain with the limbs were damaged, as all those who suffer spinal cord injuries bear witness to. This shows that this supposedly incorporeal mind can't control the actions of the body directly, meaning that it would exert the control at the brain level. The soul, in turn, is usually considered as something more emotional and less rational than what is generally known as the so-called "mind", but human emotions can also cause, for example, the heart to beat faster, the face to blush, or even some tears to escape from our eyes. In both cases we find ourselves facing an entity that triggers observable responses in the body it is associated with. And how could something metaphysical cause physical responses? We need some kind of interaction. This mind or soul has to interact in some way with our nervous system to be able to cause an effect on it.
But this is, precisely, what a measurement consists of. In Physics, in order to measure the object being studied, it has to interact in some way with the measurement device, be it the camera on a telescope, a radio antenna, a thermometer, a gravimeter, one of the many detectors in a particle collider, or even the retinas in our eyes or the neurons in our fingers. Without a physical interaction, none of the information the soul or mind could generate can be transmitted to the body —neither a message from some god or spirit, nor a premonition from the future, not even a simple thought. Therefore, if any event of this kind can happen, the absolute impossibility of being measured can't be a characteristic of the entity that causes it. And this in turn implies that, since it has to necessarily undergo a physical interaction, this entity is something also physical and not belonging to another kind of "reality". Since an entity that doesn't belong to the physical world can cause absolutely no alterations on it, it could be considered that for all practical purposes, something like this doesn't exist.
However, could a mind- or soul-like entity actually exist? It has to be said that until today no measurements or experiments have suggested its existence (no, the "aura photography" that was popular years ago doesn't count since its fraudulent nature is proven). But could it be that our technology isn't advanced enough to detect it? It wouldn't be a first. To show some examples, the subatomic particle known as neutrino couldn't be detected until 26 years after its existence was postulated in 1930, and something similar happens with other already confirmed particles (like the top quark) or even the ones that are nowadays searched for in experiments like the LHC to confirm or discard theoretical models. Nevertheless, there are important reasons why these particles were hard to detect. In the case of neutrinos, we find that they interact so weakly with matter that if we pointed a beam of them to a wall made of lead, we would need a thickness of more than a light year for it to be able to stop just half of the neutrinos that traveled through it. Other particles, like the top quark, are so heavy that are only created in processes so energetic that they would have… adverse effects if they happened routinely in our body. So if any energetic particle that transports information from the mind or the soul to our brain (and vice-versa) actually existed, it would have to interact enough with matter to cause effects in this organ, and we should be able to produce it at energies much lower than the maximum limit of our accelerators. Something like that would in principle have already been detected some time ago. And if the interaction between soul/mind and brain belongs to a kind completely new and unknown to science, the equations that describe all particle physics would have to be altered in a way that everything we've known about matter until now would be contradicted, as Sean Carroll explains well in this detailed post. Since these equations describe with precision everything that happens in everyday scales, it doesn't seem like a viable option.
But there's a chance remaining. Maybe the soul/mind can't be measured with common instruments because it interacts specifically with the neurons in the brain. That it only affects these, would explain the impossibility of measurement mentioned earlier, but some interesting details emerge here. Indeed, if there exists a soul or mind separate from the body, it seems that its relation to neurons is incredibly close. Decades of research on brain damage injuries have shown how, depending on the region of the brain they affect, cognitive abilities or even the personality can be drastically altered. If all or at least part of these characteristics belong to the soul or the mind, then it seems that this one gets modified according to the changes in brain structure. Even inserting hormones, medicines or drugs can have an effect on the abilities or feelings it expresses in each moment. It is as if this mind or soul was a faithful reflection of neuronal activity and how it is affected by internal or external agents. And if we have it this way, wouldn't we have to consider that the soul or mind could just be the result of the coordinated actions of millions of neurons in an extremely complex organ through purely biological processes, instead of invoking a supernatural entity as an explanation? In view of the objections exposed along this text, it seems the most reasonable option.
Throughout my life, I have been trying harder each time to make my beliefs have a basis as solid as possible. To question what one believes, and to be able to explain why one thinks such things, are essential tools to be able to get rid of biases when making interpretations and taking decisions that can be very relevant in the path one chooses to follow in life, so I try to apply them whenever is possible. What can be read here are the arguments that make me not believe in the existence of a mind or soul as a supernatural entity separate from the body it is normally associated with. Of course, they're subject to change when faced with evidence of their inaccuracy if that evidence is reasonable. And since a huge amount of people have an opinion about this issue which is contrary to mine, I think it can be useful to leave them expressed here. All criticism is welcome.