The first few chapters of The Study in Scarlet, the first book in The Adventures of Sherlock Holmes, is when Dr. Watson and Mr. Holmes meet each other for the first time. Much of Holmes’ character and his philosophy on life and knowledge is revealed through these interactions.

When Watson finally discovered Holmes’ profession, he didn't know exactly what made Sherlock a better detective than anyone else.

Holmes’ answer:

I’m generally able, through my knowledge of the history of crime, to set [mysteries] straight. There is a strong family resemblance about misdeeds, and if you have all the details of 1,000 at your finger ends, it is odd if you can’t unravel the 1,001.

Sherlock also said: “I have a lot of special knowledge which I apply to the problem, and which facilitates matters wonderfully.”

But Sherlock didn’t just go around cramming his head full of facts; he only remembered what would be useful for his “application.” When Watson first learned this, he was astonished:

(Sherlock Holmes is written from the perspective of Watson.)

His ignorance was as remarkable as his knowledge. Of contemporary literature, philosophy and politics he appeared to know next to nothing. Upon my quoting Thomas Carlyle, he inquired in the naivest way who he might be and what he had done. 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. That any civilized human being in this nineteenth century should not be aware that the earth travelled round the sun appeared to be to me such an extraordinary fact that I could hardly realize it.

“You appear to be astonished,” he said, smiling at my expression of surprise. “Now that I do know it I shall do my best to forget it.”

“To forget it!”

“You see,” he explained, “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 skillful 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.”

(Holmes’s “attic” quote is one of my favorites in any fiction book—nay, of any book.)

To help himself understand what Sherlock knows and doesn't know, Watson writes this telling assessment of the “limits” to Holmes’ knowledge:

SHERLOCK HOLMES—his limits.

1. Knowledge of Literature.—Nil.
2. Philosophy.—Nil.
3. Astronomy.—Nil.
4. Politics.—Feeble.
5. Botany.—Variable. Well up in belladonna, opium, and poisons generally. Knows nothing of practical gardening.
6. Geology.—Practical, but limited. Tells at a glance different soils from each other. After walks has shown me splashes upon his trousers, and told me by their colour and consistence in what part of London he had received them.
7. Chemistry.—Profound.
8. Anatomy.—Accurate, but unsystematic.
9. Sensational Literature.—Immense. He appears to know every detail of every horror perpetrated in the century.
10. Plays the violin well.
11. Is an expert singlestick player, boxer, and swordsman.
12. Has a good practical knowledge of British law.

Sherlock puts it another way:

It is of the highest importance in the art of detection to be able to recognize, out of a number of facts, which are incidental and which vital. Otherwise your energy and attention must be dissipated instead of being concentrated.

How to learn like Sherlock Holmes and become great in your field

1. Figure out what subjects matter to your life’s “application.”
2. Learn everything you can about those subjects. Learn also everything you can about the subjects once or twice removed from those subjects.
3. Learn the history of your field better than anyone else so you can determine patterns that no one else sees.

Bonus knowledge about Sherlock Holmes and Sir Arthur Conan Doyle

What’s fascinating is that Holmes was based on one of Sir Arthur Conan Doyle’s medical professors:

In 1876, Doyle enrolled as a medical student at the University of Edinburgh, where he met the man who would change his life. On his first day in Dr. Joseph Bell’s class, Doyle watched the professor hold up a vial filled with amber liquid. This, gentlemen, contains a most potent drug,” Bell said. “It is extremely bitter to the taste. Now, I wish to see how many of you have developed the powers of observation that God granted you.” Opening the vial, Bell stirred the liquid with his finger. “As I don’t ask anything of my students which I wouldn’t do alone with myself, I will taste it before passing it around.” He licked his finger and grimaced at the seemingly unpleasant taste. Then he passed the vial around. Would Doyle meet the challenge? When it was his turn, he tasted the liquid. But neither Doyle nor his classmates proved to have what Bell was looking for. “Gentlemen,” the professor said, “I am deeply grieved to find that not one of you has developed his power of perception, the faculty of observation which I speak so much of, for if you had truly observed me, you would have seen that, while I placed my index finger in the awful brew, it was my middle finger—aye—which somehow found its way into my mouth.