What is the importance of facts in Sherlock Holmes's detective work?

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Chapter 2 of A Study in Scarlet is titled "The Science of Deduction," so it is not surprising to find there some important insights about Holmes's method. In this chapter, Watson, when first getting to know Holmes and trying to figure out what profession Holmes is engaged in, is surprised by the peculiar and uneven nature of the other's expertise:

within eccentric limits his knowledge was so extraordinarily ample and minute that his observations have fairly astounded me . . . .

Holmes is extremely knowledgeable about certain, often startlingly strange, subjects. (He can, for example, identify where in London he got different splashes of mud after going for a walk, and his ability to tell the difference between types of tobacco ash comes into play in several stories.) But regarding other, often very common, things:

His ignorance was as remarkable as his knowledge . . . . My surprise reached a climax, however, when I found incidentally that he was ignorant of the Copernican Theory and of the composition of the Solar System.

It turns out that Holmes has a particular theory of mind and memory that has led him into this state of knowledge. He says,

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.

It is not only the facts of each individual, specific case that are central to Holmes's deductive method (see the previous answer). This data requires an expansive and detailed set of general facts, accumulated and stored as the background knowledge from which Holmes draws in order to interpret the observations, clues, and details of each case. Thus, what Holmes considers worth learning is entirely dictated by his work as a detective, as odd as some of his choices may seem:

“But the Solar System!” I protested.

“What the deuce is it to me?” he interrupted impatiently; “you say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or to my work.”

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In "A Scandal in Bohemia," Holmes tells Watson:

“I have no data yet. It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”

Holmes frequently complains in other stories that he needs more facts, or more data. For example, in "The Adventure of the Copper Beeches" he says:

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

This emphasis on facts to fuel his thinking seems intended to explain why Sherlock Holmes, usually accompanied by Dr. Watson, will go out into the field to investigate for himself. Readers of the Sherlock Holmes stories expected some adventure as well as an introduction to strange characters and interesting settings. It is because of Holmes' need for facts, or data, that he leaves Baker Street for bizarre, charming, mysterious, or unusual settings where he invariably encounters adventures.

Although the detective is supposed to be a superior type of thinking machine, Sir Arthur Conan Doyle must have known that his readers could only be intrigued with a few examples of his character's deductive powers and were eager to be transported in their imaginations into the outer Victorian world where they could feel excitement and other strong emotions. Holmes is not looking for excitement but needs to see things for himself, to interview people, and to take occasional risks, as he does, for example, in The Hound of the Baskervilles.

Even when a new client has a long tale to tell him at Baker Street, it can be seen that Sherlock Holmes is trying to gather facts. For example, when Helen Stoner comes to him in the early morning in "The Adventure of the Speckled Band," he interrupts her numerous times to focus on something of a factual nature.

“Pray be precise as to details,” said he.
“One moment,” said Holmes, “are you sure about this whistle and metallic sound? Could you swear to it?”

The search for more facts will lead him to Helen Stoner's home at Stoke Moran, where he and Watson will become involved in one of their more harrowing adventures.

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