“I had,” said he, “come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data.”

“I should have more faith,” he said; “I ought to know by this time that when a fact appears opposed to a long train of deductions it invariably proves to be capable of bearing some other interpretation.”

“Men die of the diseases which they have studied most,” remarked the surgeon, snipping off the end of a cigar with all his professional neatness and finish. “It’s as if the morbid condition was an evil creature which, when it found itself closely hunted, flew at the throat of its pursuer. If you worry the microbes too much they may worry you. I’ve seen cases of it, and not necessarily in microbic diseases either. There was, of course, the well-known instance of Liston and the aneurism; and a dozen others that I could mention.”

“There’s no need for fiction in medicine,” remarks Foster, “for the facts will always beat anything you fancy.”

[Colonel Ross:] “Is there any point to which you would wish to draw my attention?”
[Sherlock Holmes:] “To the curious incident of the dog in the night-time.”
“The dog did nothing in the night-time.”
“That was the curious incident.”

[Sherlock Holmes:] The temptation to form premature theories upon insufficient data is the bane of our profession.

A man loses his fortune; he gains earnestness. His eyesight goes; it leads him to a spirituality... We think we are pushing our own way bravely, but there is a great Hand in ours all the time.

A strange enigma is man! Someone calls him a soul concealed in an animal.

Data! Data! Data! … I can’t make bricks without clay!

Detection is, or ought to be, an exact science, and should be treated in the same cold unemotional manner. You have attempted to tinge it with romanticism, which produces the same effect as if you worked a love-story into the fifth proposition of Euclid.

Do you remember what Darwin says about music? He claims that the power of producing and appreciating it existed among the human race long before the power of speech was arrived at. Perhaps that is why we are so subtly influenced by it. There are vague memories in our souls of those misty centuries when the world was in its childhood.

From a drop of water a logician could predict an Atlantic or a Niagara without having seen or heard of one or the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it.

His [Sherlock Holmes] ignorance was as remarkable as his knowledge. … he was ignorant of the Copernican Theory and of the composition of the Solar System. … “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.”

How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?

I am glad of all details … whether they seem to you to be relevant or not.

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.

I could not help laughing at the ease with which he explained his process of deduction. “When I hear you give your reasons,” I remarked, “the thing always appears to me to be so ridiculously simple that I could easily do it myself, though at each successive instance of your reasoning I am baffled, until you explain your process. And yet I believe that my eyes are as good as yours.”
“Quite so,” he answered, lighting a cigarette, and throwing himself down into an arm-chair. “You see, but you do not observe. The distinction is clear. For example, you have frequently seen the steps which lead up from the hall to this room.”
“Frequently.”
“How often?”
“'Well, some hundreds of times.”
“Then how many are there?”
“How many! I don't know.”
“Quite so! You have not observed. And yet you have seen. That is just my point. Now, I know that there are seventeen steps, because I have both seen and observed.”

I never guess. It is a shocking habit—destructive to the logical faculty.

It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.

It is a capital mistake to theorize before you have all the evidence. It biases the judgment.

It is a great thing to start life with a small number of really good books which are your very own.

It seems very strange … that in the course of the world’s history so obvious an improvement should never have been adopted. … The next generation of Britishers would be the better for having had this extra hour of daylight in their childhood.

One’s ideas must be as broad as Nature if they are to interpret Nature.

When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge.

When we think how narrow and devious this path of nature is, how dimly we can trace it, for all our lamps of science, and how from the darkness which girds it round great and terrible possibilities loom ever shadowly upwards, it is a bold and a confident man who will put a limit to the strange by-oaths into which the human spirit may wander.

Winwood Reade … remarks that while a man is an insoluble puzzle, in the aggregate he becomes a mathematical certainty. You can, for example, never foretell what any one man will do, but you can say with precision what an average number will be up to. Individuals vary, but percentages remain constant. So says the statistician.

The one [favourite fictional scientist] I can think of is Conan Doyle’s Professor Challenger, but he was a very irascible character and not a good role model.