Many “doctors of psychological medicine” in the nineteenth century wrote of patients who were able to suppress delusional notions, often for quite long spells, and claimed that this was why many of them appeared, to non-medical folks, to be sane. The doctors even devised a name for this condition: insania occulta. Sceptics replied that this caught patients in a double bind: if they expressed delusions, they were declared insane; if they failed to express delusions, they were deemed to be insane and cunning.

One of the anecdotes that did the rounds in legal and medical circles for many years harked back to the prosecution in the eighteenth century of Dr John Monro of Royal Bethlehem Hospital by one Mr Wood. Monro had certified Wood insane and had him incarcerated. Mr Wood, a merchant of the City of London, sued the doctor for false imprisonment. For hours in the courtroom, Wood spoke in a calm and rational way, recalling precise details of events and conversations. It looked as though Monro was about to lose the case.

At this point, one of Monro’s legal team whispered into the ear of the barrister questioning Wood, advising the barrister to ask Wood a very specific question – namely, “What became of the princess with whom you were corresponding in cherry juice?” Instantly, according to one who was there in court, “Wood showed in a moment what he was. He assured the court that because he was at that time, as everybody knew, imprisoned in a high tower, and being debarred the use of ink, he had no other means of correspondence but by writing his letters in cherry juice, and throwing them into the river which surrounded the tower, where the princess received them in a boat.”

This onlooker continued: “There existed, of course, no tower, no imprisonment, no writing in cherry juice, no boat, but the whole inveterate phantom of a morbid imagination.” 

Dr Monro was immediately acquitted, and the Wood case was repeated often as a warning against highly plausible “lunatics”, who needed to be questioned on specific matters in order for a torrent of delusionality to begin to flow.

Dr John Charles Bucknill wrote in 1857 of patients of his who for hours at a time would appear fully rational, except when certain subjects were touched upon. One barrister under Bucknill’s asylum care never said anything irrational, yet would sit up all night writing reams of letters boasting of his (non-existent) huge wealth, his yacht voyages to the ends of the earth and his marriage to several servant girls.

Bucknill said that night-time often called forth whole worlds of delusional fantasy in men who by day seemed to have no psychological problems: “I have for several years had under my care a respectable tradesman, whose conduct and conversation during the day exhibit scarcely a trace of mental disease. He is industrious, sensible and kind-hearted, and it is strange that his nights of suffering have left no painful impression on his pleasing features. At night he sees spectres of demons and spirits, at which he raves aloud and prays with energetic fervour.”

Dr Lyttleton Stewart Forbes Winslow had, in the 1870s, at his West London asylum, Sussex House, a “homicidal maniac” (Winslow’s words), who had made two attempts upon his wife’s life. The man used abusive language about her constantly while at Sussex House at night-time; but during the day he was able to suppress his delusions and appeared to be a contrite and loving husband.

Dr Winslow also had captive a man who seemed sane by day but who spent his nights writing graphic and obscene letters to his family, his acquaintances and high-profile strangers (all were intercepted and destroyed by Winslow).

It would be interesting to know how much, if anything, Robert Louis Stevenson knew of such cases of daytime sanity and night-time raving. Works of genius such as The Strange Case of Dr Jekyll and Mr Hyde cannot, and should not, be pinned down to a single inspiration, but it is tempting to think that this sort of discussion in psychological-medical publications may have played some small role in the creative spark.