"This agency stands flat-footed upon the ground, and there it must remain. The world is big enough for us. No ghosts need apply."
-- Sherlock Holmes, "The Adventure of the Sussex Vampire"
This is just what we might expect to from the supremely rational Sherlock Holmes, a man who even applied deductive reasoning to what he considered to be the science of religion. (We see this in "The Naval Treaty" where he finds in the existence of roses an assurance of the goodness of Providence.)
And yet --
As I have noted before, the Canon is replete with situations that at first appear to be supernatural. Like his idol Edgar Allen Poe, Sir Arthur Conan Doyle wrote both horror stories and detective stories that often were tinged with horror.
The current expert on this subject seems to be Tracy Revels. On her wonderful blog Strictlty Sherlock, the author of Shadowfall is writing a great series on "Haunted Holmes." So far she has covered "The Crooked Man," "The Adventure of the Devil's Foot," and "The Adventure of the Speckled Band," with the happy promise of more to come.
Shadowfall is just the kind of book I don't like -- but I did! It's well written, evocative, and highly imaginative. Somehow it worked for me even though it takes us well beyond the Canonical Holmes.