You see, I consider that a man’s brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skilful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.

Arthur Conan Doyle (1859-1930) British writer and physician

Story (1886-04), “A Study in Scarlet,” Part 1, ch. 2 [Holmes], Beeton’s Christmas Annual, Vol. 28 (1887-11-21)
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Connected to this passage.

Published in novel form 1888-07. See also "The Five Orange Pips."

In the Sherlock TV episode 01x03 "The Great Game" (w. Mark Gatiss) (2010-08-08), this explanation is reworked:

SHERLOCK: Listen: (points to his head) This is my hard-drive, and it only makes sense to put things in there that are useful. Really useful. Ordinary people fill their heads with all kinds of rubbish, and that makes it hard to get at the stuff that matters! Do you see?