‘Deficit’ is neurology’s favourite word – its only word, indeed, for any disturbance of function. Either the function (like a capacitor or fuse) is normal – or it is defective, or faulty: what other possibility is there for a mechanistic neurology, which is essentially a system of capacities and connections?

What then of the opposite – an excess or superabundance of function? Neurology has no word for this, because it has no concept. A function, or functional system, works – or it does not: these are the only possibilities it allows. Thus a disease which is ‘ebullient’ or ‘productive’ in character challenges the basic mechanistic concepts of neurology, and this is doubtless one reason why such disorders – common, important and intriguing as they are – have never received the attention they deserve. They receive it in psychiatry, where one speaks of excited and productive disorders – extravagances of fancy, of impulse . . . of mania. And they receive it in anatomy and pathology, where one speaks of hypertrophies, monstrosities, of teratoma. But physiology has no equivalent for this: no equivalent of monstrosities or manias. And this alone suggests that our basic concept or vision of the nervous system – as a sort of machine or computer – is radically inadequate, and needs to be supplemented by concepts more dynamic, more alive.

This radical inadequacy may not be apparent when we consider only deficits or loss, but it becomes immediately obvious when we consider, as we do here, their excesses, for these are characterized almost violently by dynamism, bursting and burgeoning. Then we see at once how traditional neurology, in its mechanicalness and its emphasis on deficits conceals from us the vivid ‘go’ of cerebral functions – at least such higher functions as those of imagination, memory and perception. It conceals from us the very life of the mind.