Not so elementary, my dear Watson!

Sherlock Holmes sometimes speaks as though no genius is required in solving crimes, that it is just a matter of applying knowledge already in one's possession:

... I have a trade of my own. I suppose I am the only one in the world. I’m a consulting detective, if you can understand what that is. Here in London we have lots of government detectives and lots of private ones. When these fellows are at fault, they come to me, and I manage to put them on the right scent. They lay all the evidence before me, and I am generally able, by the help of my knowledge of the history of crime, to set them straight. There is a strong family resemblance about misdeeds, and if you have all the details of a thousand at your finger ends, it is odd if you can’t unravel the thousand and first ...

--A Study in Scarlet

The point Sherlock Holmes seems to be making here is that solving crimes is merely the application of knowledge already in our possession. He seems to have overlooked the fact that sometimes the knowledge required to locate clues and to make sense of them has yet to be discovered. Kasiski discovered the one weakness that all polyalphabetic substitution ciphers share. Before his discovery polyalphabetic substitution ciphers were thought to be unbreakable.

In Black Peter, to determine who the murderer was, Sherlock Holmes first had to make good a piece of knowledge he found he lacked. He spent a whole morning driving a harpoon again and again into a pig carcass to see what it was like. As a result he came to the right conclusion as to who killed Black Peter. This is to say, if Sherlock Holmes had been more careful he also would say that in following clues we cannot always rely on knowledge we already have; sometimes we need more; sometimes we even have to find that missing piece of knowledge for ourselves, doing so as a sidetrip to the main investigation.




(See also 'Is Sherlock Holmes a genius? and 'Yes Sherlock Holmes is a genius!'.)