This is a short story, situated in the near (third or fourth millennium) future.
An absolutely brilliant mathematician is working on a unified system of mathematics. One that ties all of the fields together in a seamless process. Just as physicists are trying to establish a unified theory, they need to develop a unified system of mathematics. That is, each branch of math must operate under exactly the same consistent rules, operators, and methods. Conjecturally for example, and I have no idea how this would work, being able to take a term on the inside of an integral to the 'other side of the equation' (or between integrals) and leave all other terms intact in the integral. This theory will revolutionize the fields of mathematics and physics. The stakes for society are high, potentially a 'crossroads' for human history.
No one can yet follow it as it is not complete. His mind must be preserved in working condition to assure his work is not lost.
Medical science has evolved to the point where all bodily functions and organs can be synthesized - heart, lungs, pancreas, kidneys, skin - everything. They have the ability to shorten neurons and attach them to artificial synapses, so the spinal chord can terminate just below the neck. They have attached these systems to his head, in lieu of his body, to ensure he (his mind) does not die. They have done so in duplicate, so if one fails, they can instantly switch to another. Using stem cells, they can keep his mind cells regenerating, so no worries about telomeres or his mind aging. He still has all sensory inputs to the brain - intact optical, smell, and hearing systems. distant sensory systems are simulated. He is attached to several different 'robots' so he can draw, write, type, manipulate objects, using the same neural pathways as the human body, through the artificial synapses.
He can eat and breathe, taste and smell, but these operations are inconsequential - to preserve normal habits. They are independent of the functions keeping his body in homeostasis. Nutrition is done automatically through the artificial organs, not through anything he does.
His mind has no control over his bodily functions. All blood hormones, proteins, neurotransmitters, and cells are controlled by a computer. All moderator levels of drugs for psychoactive states (anxiety, depression, mood) are computer monitored and maintained at optimum levels. Heart rate, breathing, blood temperature, all done through computer control. He is kept in perfect hormonal and nutrient balance automatically. The computer imitates the normal circadian rhythms. The hope is to keep his mind alive and conscious as long as it takes to complete the theory - a decade, five decades, a century, two or three centuries.
Here is the thing. For plot purposes, he dies. That is, his mind no longer has any consciousness. He no longer 'thinks'. Absolutely nothing has changed about the functioning of the control system. Hormones, energy, protein, oxygen levels, have all been consistently maintained. One second, everything is operating normally, and he is conscious. Next second, everything is operating normally, but he is no longer conscious - his mind is not 'alive' in any sense but background electrical impulses maintained by the biological machine. If you have been beside someone at the point of death, it is exactly the same thing. Alive one second, gone the next.
So the question is, except for handwaving the death away, or reverting to religious concepts of the soul, is there any biological physiological way to explain HOW the mind just gave up, lost all consciousness, and effectively 'died', even though all support systems are intact and properly functioning? What would the biological state change in his mind be (that his mind could still control - as in 'will to NOT live') between the second his mind was fully artificially physiologically functioning and alive and the next second when his mind was fully artificially physiologically functioning and dead?
This is not about committing suicide. This is not about self-destruction. It is not about mechanical malfunction or slow degeneration. It is not even really about life and death. This is about controlling one's self, and one's brain. It is about the ultimate 'I WILL not...' but society saying 'You WILL...' decision, with very high stakes.
I am looking for an answer:
that does not involve equipment failure or malfunction.
that does not involve intervention by another person or machine.
that can be controlled or moderated solely by the mind of the person, including all of the peripheral glands immediately attached to the mind that can be modified under the control of the mind.
that could include proven scientifically referenced ideas such as meditation, deep mind control of physiological mental states, and manipulation of biofeedback to control brainwaves.
that does not result in the physical destruction of the brain, but in the irreversible cessation of brain activity (deeper than a self-induced coma).
I am looking for hard evidence from medical science where they are investigating similar situations. For example, think perhaps of the elderly couple, devoted to each other, in the very last stages of their lives. One dies, and the other dies shortly thereafter. No absolute medical reason why, except that the mind decided that it was time to go. However, in this case, it can not be attributed to the body that shuts down, it is the mind.
There is a lot in speculative metaphysics, but I am after something scientifically and medically supportable.