“Elementary, my dear Watson,” says Sherlock Holmes.
And, since it’s not elementary, but rather occult and exoteric, the reader is apt to feel annoyed and to think the expression pure ostentation on Sherlock’s part. However, it isn’t. Detectives all talk that way. Doyle is realistic, after all. Even William J. Burns, who has less in common with Sherlock Holmes than any detective ever born or invented since Lecocq and Dupin, looks surprised when you ask him how he did it, and says — well, he doesn’t say “elementary,” but he says:
“Just common sense.”
BURNS, A DETECTIVE FROM WHOM LECOQ MIGHT LEARN: Astonishing Record of a Great Sleuth Who Has Been Employed on All of Uncle Sam’s Big Cases for a Generation, and Who Now Claims to Have Caught the Los Angeles Dynamiters. (PDF)
From April 30, 1911