Some months after Sherlock Holmes and I set up housekeeping together at the instigation (on my part) of the Ministry of Magic, I was carried into the case which has been published in the Muggle press as "A Study in Scarlet". At the end of that case, I felt confident in reporting definitively to the Ministry that Sherlock Holmes' discretion and moral sense could be firmly relied upon. Accordingly, the decision was made to acquaint him with the existence of the wizarding world, as he had already deduced the existence of magic and of its individual practitioners. I fear he did not entirely believe my feeble attempts at elucidation, and unfortunately as a Squib I was unable to provide more direct evidence. However, a personal visit from the Minister of Magic was more convincing. That was the beginning of an occasional but fruitful collaboration between Holmes and the Ministry, in which he was able to provide them solutions to a few puzzling problems. Since wizard gold is not useful in the Muggle world, Holmes was repaid with a few small magical instruments that proved particularly useful in his unique profession.
The cases in which Holmes aided the Ministry of Magic can of course never be published in the Muggle press; however, some of them cast such a unique light into my friend's methods that I have determined to recount them in the periodicals that circulate within the Wizarding world.
I have received a number of letters from Muggles asking where they might acquire copies of Holmes' monographs mentioned in my stories, discussing the identification of types of soil or tobacco ash. I fear those letters will never receive a reply, as the monographs do not in fact exist. As the astute Wizard reader will have surmised, those identifications were performed by magic rather than logic. Indeed, Holmes' researches determined that the consistency of tobacco ash depends far more on external conditions than on the type of tobacco used. He reluctantly concluded that identification of tobacco from the ash was not physically possible. While the composition of soil can indeed be determined through chemical assay, the instant visual identification of its provenance with which I have credited my friend is also somewhat outside the range of normal human powers. Though he seemed to have some internal struggle with the idea of using magic rather than logic, I believe he concluded that the result of identifying criminals permitted a slight deviation from the strict adherence to strict logic in that identification.
"Of course, Watson," he once reasoned, "Since it was by the use of deductive logic that I originally surmised the existence of true magic, it may be said that I have not actually transgressed the rules of the game."
After we shared the excellent suppers brought up by the estimable Mrs Hudson, Holmes was wont to retire for the evening with one of the books of magical lore he had been wont to study since taking on Wizarding cases. He purchased them at Flourish & Blott's; I had never seen such a look of unguarded pleasure on his face as when he realized that there, in Diagon Alley and its surroundings, was an entire section of London of which he had been previously unaware. He spent some evenings absorbed in updating his notes, adding the wizarding neighborhoods to his encyclopaedic knowledge of the city. (The publican at The Leaky Cauldron conveyed him through the portal to Diagon Alley, as he had always done for Muggle parents of Hogwarts' students.)
I had voiced my surprise at the breadth of these studies, when he began. Upon our first meeting, Holmes had informed me that he preferred not to clutter the "lumber room of his mind" with knowledge which could not help in his profession; thus I was much taken aback to observe him poring over texts of mythical creatures and legendary tales as well as sober texts on more factual subjects such as Transfiguration or Arithmancy.
Holmes replied, drily, "Your Wizarding society is not precisely the same as the wider British milieu, Watson. To understand a society's crimes, one must first understand its character, which can be usefully assessed through its stories. The difference between Wizarding and Muggle Britain, for instance, shows up particularly clearly in the different perspectives on those legendary figures who figure in both traditions, such as King Solomon or Merlin."
His strategy appeared to be successful, in that his comments on the crime cases recounted in the pages of the Daily Prophet were proved correct as invariably as those on more mundane crimes reported in the three other newspapers he read daily.
Some weeks before the case which I have recounted in the Muggle press under the title "The Valley of Fear", our old friend Tobias Gregson dropped by one stormy evening to smoke a pipe with us. The housekeeper showed him to our rooms, then took his overcoat down to dry by the kitchen fire.
"It's not a fit night for man nor beast," remarked the Inspector, as he stood held his hands to the fire to warm them.
Holmes replied with a twinkle, "Is it a night for Scotland Yard, then?"
"Very funny, Mr Holmes," said the inspector, without any evident humor. "We civil servants haven't the luxury of choosing our cases, you know. In fact, you may have heard about this series of crimes– the popular press are calling it the Jack the Ripper case."
"I have indeed. A particularly unpleasant series of murders, perpetrated upon ladies of somewhat uncertain virtue."
"That's right. None of the sort of mystery that interests you gentlemen, just some ugly killings and straightforward police work, though there are a few odd angles. We'll get the brute in the end, just wait and see."
"I have no doubt you will," said Holmes graciously, and we went on to speak of other matters.