"My mind," he said, "rebels at stagnation. Give me problems, give me work, give me the most abstruse cryptogram or the most intricate analysis, and I am in my own proper atmosphere. I can dispense then with artificial stimulants. But I abhor the dull routine of existence. I crave for mental exaltation. That is why I have chosen my own particular profession,—or rather created it, for I am the only one in the world."

Sherlock Holmes says this to Dr. Watson at the beginning of the book. The detective craves the excitement of cracking cases through a process he calls "deduction"—using a mass of evidence accumulated through keen observation to solve a mystery. He is, in fact, the only detective in the world to use this process, and he is in a class by himself. However, when in between cases, he feels at loose ends. He takes cocaine to soothe himself at these times, and Dr. Watson finds this practice both intriguing and revolting, as Watson himself does not use the substance.

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

This is Holmes's pronouncement on the science of detection. He treats it as a science, and he is cold and rational in his decision-making. Watson, however, is more romantic, and Holmes criticizes this tendency in Watson. Holmes says that interjecting fanciful notions into the calculated empiricism of detection is like introducing a love story into the geometry developed by Euclid. Indeed, Holmes is successful because he is so rational and detached.

"You really are an automaton,—a calculating-machine!" I cried. "There is something positively inhuman in you at times."

Watson says this to Holmes when Watson observes that their client, Miss Morstan, is quite attractive. Holmes replies that he did not notice her looks at all, which shocks Watson. In Watson's mind, Holmes’s way of thinking and acting is strange and almost inhuman. However, it is Holmes's detachment and scientific mind that makes him a superlative detective, because he is not misled by emotions and is an excellent observer of facts.

"In the left-hand corner is a curious hieroglyphic like four crosses in a line with their arms touching. Beside it is written, in very rough and coarse characters, 'The sign of the four,—Jonathan Small, Mahomet Singh, Abdullah Khan, Dost Akbar.'"

Holmes comments on the strange paper from Miss Morstan's father's desk. He realizes that this letter, which has been apparently kept in a pocketbook, is of great importance to the case. He comes to unravel the mystery of this letter, including who these four men are and what the map in the letter represents: the site of the burial of the treasure.

"Whoever had lost a treasure, I knew that night that I had gained one."

When Watson realizes that the treasure that Miss Morstan had stood to inherit is gone, he feels only a sense of relief. This great treasure would have remained between the pair, and he loves Miss Morstan and wants to marry her. He regards her as a greater treasure than the famed treasure from India that her father was supposed to gain. This shows that Watson is a sentimental person, unlike Holmes.