That Latimer/Spencer suffers from temporal lobe epilepsy generates a bundle of perplexing questions. When Eliot published The Lifted Veil in 1859, this condition was hardly on the medical landscape in the British Isles. It would not enter the medical literature there until the period 1876-1888 when the brilliant neurologist John Hughlings-Jackson published his now classic descriptions of the syndrome. If nothing else the author of TLV is a superbly well-informed lay person who is nothing short of prescient about this species of epilepsy.
So how and where did George Eliot get all the epilepsy stuff? To this question there is, I think, a fairly simple answer. As suggested above, the superbly well-informed lay person who wrote the novella was actually two people--George Eliot and George Henry Lewes. Surely Lewes, who would go on in the 1870s to write at length about epilepsy in his own impressive Problems of Life and Mind, shared with her all that he knew about the condition. That's the easy part.
A more complicated question has to do with the somewhat unnerving overlap between what Eliot/Lewes made of what we would call temporal lobe epilepsy in 1859 and what Hughlings-Jackson will do with it beginning in the mid-1870s. For example, the telling phrase "double consciousness" stands at the center of both the fictional and the extra-fictional renditions.
Now we could say that the resemblances between the fictional picture of the temporal lobe epileptic published in Blackwood's and the extra-fictional one that Hughlings-Jackson began publishing almost 20 years later derive from a series of coincidences. But some curious continuities tie together the two portraits in question. Lewes, who schooled George Eliot in the epileptic, developed a friendship with Hughlings-Jackson in the early 1870s and held a series of "medical chat[s]" with him. Indeed the neurologist became for a season a regular at the famous Sunday gathering at the Priory, Eliot and Lewes' home. Further, the figure of Spencer himself ties the two pictures together: in the late 1860s and 1870s he drew close to Hughlings-Jackson. The clinician went so far as to claim Spencer as a major intellectual inspiration.
The deep irony involved in that claim has never been perceived. A disinterested look at the evidence suggests that Hughlings-Jackson, whom I cannot resist calling a true genius, had very little to learn from the plodding Spencer qua philosopher. No: his debt to Spencer resides elsewhere. He cannot but have learned of Spencer's secret illness and drawn upon it for his own portrayal of the temporal lobe variant of epilepsy. And it must have been Lewes who introduced this aspect of Spencer to the brain specialist. Thus the ever-versatile G. H. Lewes served as midwife both to George Eliot's Latimer and to Hughlings-Jackson's temporal lobe epileptic.
However counter to the orthodoxy that reigns in the history of the neurosciences, this rather surprising conclusion is the only one that does justice to the evidence.