Francis Crick wrote in his book on the brain "The Astonishing Hypothesis" (1994):
"You", your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will are in fact no more than the behaviour of a vast assembly of nerve cells … As Lewis Carroll’s Alice might have phrased it: "You’re nothing but a pack of neurons".
This belief arises from the idea that the neurons are obviously busy doing stuff - sending signals, interacting with each other, etc. - and nothing else special appears to be going on in the cranium, so "the behaviour of a vast assembly of nerve cells" has to explain everything. Our consciousness and identity, mental states & events - the lot.
This approach may appeal to some - in the absence of a viable alternative: And no serious thinker now believes that we are "funny magic moonbeams" in our heads somehow influencing the material world ("swerving atoms"). So what else can the mind be? what else is there?
From this model of the human mind as the "the behaviour of a vast assembly of nerve cells", we may infer that the wiring & components of the brain must be essential to the mind. After all, consider an analogy: the electronic components of a computer must be assembled according to a fixed plan - you can't jumble them up, or lose half of them - so surely the brain, and therefore the mind, must develop according to a very detailed plan, or fixed rules. The brain, and therefore the mind, will have a very precisely ordered structure - and it would be extremely unlikely to be improved by random changes. In which case, if we accept this analogy & reasoning, then we would expect that individuals with very different & unusual brains will surely have drastically different and - let's face it - impaired mental functions.
And that all might seem to make sense - except that there are individuals who live their lives with no obvious impediment, even though they have very abnormal brains. They show that the brain isn't the same as a computer, and that the mind isn't the same as the brain.
A 44 year-old man presented with a 2-week history of mild left leg weakness. At the age of 6 months, he had undergone a ventriculoatrial shunt, because of postnatal hydrocephalus of unknown cause. When he was 14 years old, he developed ataxia and paresis of the left leg, which resolved entirely after shunt revision. His neurological development and medical history were otherwise normal. He was a married father of two children, and worked as a civil servant.
Massive ventricular enlargement, in a patient with normal social functioning
(A) CT; (B, C) T1-weighted MRI, with gadolinium contrast; (D) T2-weighted MRI. LV=lateral ventricle. III=third ventricle. IV=fourth ventricle. Arrow=Magendie's foramen. The posterior fossa cyst is outlined in (D).
Further desciption of unusual brain cases can be found here: