Descartes Error is written by a neuroscientist who studied traumatic brain injury patients and found people who could perfectly pass an IQ test and apparently "function," but were completely incapable of making 'rational' decisions, staying focused, exerting self-control, and accurately understanding social situations.
TLDR their emotional capacities were damaged, and made it impossible to be "logical" or "rational." We are only capable of reasoning through emotion.
Thus the title 'Descartes Error' alludes to Descartes originally claiming emotion and reason were completely separate, forming the basis of the way we still talk and think about it. Eg. "don't let your feelings influence your decision"
In 1848 a railroad construction worker had half his head blown off by an iron rod in a poorly controlled explosion. Surprisingly, he is still able to walk and talk and function as before. He only loses vision in one eye. But It transforms his entire personality.
He goes from well respected intelligent business man to an obstinate, socially inappropriate creature unable to make decisions or stick to long term plans, and throws childlike tantrums regularly.
Essentially, he was unable to Reason. It was the first suggestion that some parts of the brain are "dedicated more to reasoning than to anything else, and in particular to the personal and social dimensions of reasoning"
Included the ability to anticipate and plan for the future , and a sense of social and personal responsibility he failed to consider ethics and social conventions. He was unable to function as a social being, despite having the language, perception, and objective intelligence parts of god brain in tact.
Similar defects have been seen in patients with brain legions
"It was difficult to accept the view that something as close to the human soul as ethical judgment, or as culture-bound as social conduct, might depend significantly on a specific region of the brain" 21
There are numerous cases of high achieving individuals who, after surgery to frontal lobe tumours, become incapable of managing a schedule, staying on task, or making reasonable decisions about what they should do and when.
They become obsessed by small details and unable to see the bigger picture. They obsessively collect things. They make irresponsible financial decisions. They enter relationships with shady characters.
We would traditionally judge these type of things as poor judgement, "lack of character" or unethical disposition.
"The tragedy of this otherwise healthy and intelligent man was that he was neither stupid nor ignorant, and yet he acted often as if he were"
Despite living with the disastrous repercussions, they are unable to learn from these mistakes.
We have to conclude their free will was compromised. The core skill they lost was simply decision making.
More interestingly, other kinds of reasoning were unaffected. They were still able to perform well on various kinds of logic tests. They could piece together unrelated bits of knowledge to arrive at educated guesses for answers. Anything resembling an IQ test was no problem.
They are only unable to make decisions In distinctly social matters. It made no sense... until we consider emotion .