In Chapter 1, Holmes and Watson discuss Holmes's detective work and his impressive ability to reason through deduction. Through these conversations, the reader is also privy to Holmes's extraordinary self-confidence. He often conveys this confidence through similes that trivialize his esoteric, highly specialized knowledge and skillset, as in the following passage:
If you can say definitely, for example, that some murder had been done by a man who was smoking an Indian lunkah, it obviously narrows your field of search. To the trained eye there is as much difference between the black ash of a Trichinopoly and the white fluff of bird's-eye as there is between a cabbage and a potato.
In this passage, Holmes uses a simile to compares the difference between different types of tobacco ash to the obvious difference between two very different vegetables. Of course, it's almost inconceivable that types of tobacco ash could be so obviously distinct from each other, but such is the power of Holmes's keen eye.
Passages like this prepare the reader for the many demonstrations of Holmes's shrewd detective skills to come—they are, after all, the driving force of the narrative, as he puts his eye to work in service of the mystery of the Indian treasure. These sorts of passages also establish one of the primary themes of the novel: the power of reasoning (and Holmes's rationality) to overcome almost any obstacle.
In Chapter 1, the reader meets the detective Sherlock Holmes and the unconventional—and highly principled—methods that he employs in his detective work. Holmes explains his philosophy of "detection" through simile and allusion:
'Honestly, I cannot congratulate you upon it. 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.'
The "fifth proposition of Euclid" is an allusion to the Euclidean geometry theory of the Ancient Greek mathematician Euclid. This particular proposition concerns how to determine the relative angles of intersecting lines. By asserting that detective work should be "cold and unemotional," Holmes compares the consideration of emotion in the act of detection to the romanticization of geometry—both, Holmes argues, would be equally nonsensical.
This is an early passage that speaks to the constant tension between the rational and the emotional throughout The Sign of the Four. While Holmes is dedicated to rationalism to the point of austerity, Watson is a highly emotional character who often lets himself get carried away by fancy. The two characters thereby function as foils to each other, drawing out the other's inherent qualities in greater intensity through relief. In that respect, Watson's role as narrator of the novel is chiefly responsible for highlighting Holmes' dogged commitment to reason above all else.
Doyle wrote The Sign of the Four at the height of the Victorian period, at the end of the 19th century, in the midst of a scientific revolution that presented rational analysis and scientific reasoning as possible solutions for life's biggest problems. Holmes is a sort of avatar for this attitude and a spokesperson for the beauty of an "exact science."
In Chapter 5, Holmes and Watson begin to unravel the mystery of Bartholomew Sholto's murder. As Holmes pieces the scene together and prepares to trace the escape of the murderers, Watson wields a double simile to compare Holmes's shrewd observation skills to those of an animal:
He whipped out his lens and a tape measure and hurried about the room on his knees, measuring, comparing, examining, with his long thin nose only a few inches from the planks and his beady eyes gleaming and deep-set like those of a bird. So swift, silent, and furtive were his movements, like those of a trained bloodhound picking out a scent, that I could not but think what a terrible criminal he would have made had he turned his energy and sagacity against the law instead of exerting them in its defense.
Holmes is so formidable, by Watson's appraisal, that he combines the best and most ruthless cunning of a bird of pray and a hunting dog. Throughout The Sign of the Four, Watson uses a range of simile and metaphor to describe—and exalt—Holmes's skill, often in terms that suggest Holmes to be a creature of pure logic incapable of emotional complexity. In Chapter 2, Watson calls him an "automaton," while this simile further suggests that the detective has the pure instinct of an animal.
Unlike an animal or an automaton, however, Holmes is capable of applying reason to his discoveries: his instinct toward observation may be innate and inhuman in its power, but he can rationalize his findings into the larger narrative of the mystery at hand. While Watson is an embodiment of emotional sensitivity in The Sign of the Four, he makes it clear to the reader that Holmes is the opposite.
In Chapter 7, the reader meets the talented Toby—a dog with a legendary nose whom Holmes and Watson recruit from a kennel in their search for the Wooden-Legged Man. Doyle uses a simile to emphasize Toby's uncanny skill at identifying and tracking scents:
He pushed the creosote handkerchief under the dog's nose, while the creature stood with its fluffy legs separated, and with a most comical cock to its head, like a connoisseur sniffing the bouquet of a famous vintage.
In this simile, Doyle compares Toby to a wine connoisseur—the way that Toby cocks his head when Holmes shows him the scented handkerchief appears similar to the tilt of a connoisseur in the act of smelling a fabulous wine. By personifying Toby in this playful way, Doyle ensures, much to the reader's delight, that the dog joins the colorful cast of The Sign of the Four as a character in his own right.
Earlier in the novel, the narrator compared Holmes to a bloodhound in his remarkable ability to doggedly follow clues as he solves his mysteries. In this case, however, Doyle flips the script and the narrator compares an actual hound to an intelligent, cultured man not unlike Holmes himself.
