I had had so many reasons to believe in my friend’s subtle powers of reasoning and extraordinary energy in action that I felt that he must have some solid grounds for the assured and easy demeanour with which he treated the singular mystery which he had been called upon to fathom. Once only had I known him to fail, in the case of the King of Bohemia and of the Irene Adler photograph; but when I looked back to the weird business of the Sign of Four, and the extraordinary circumstances connected with the Study in Scarlet, I felt that it would be a strange tangle indeed which he could not unravel.