Deductive/abductive reasoning
Holmes' success in solving crimes stems from his ability to analyze small pieces of evidence and draw inferences from them. Abductive reasoning is a more accurate way of describing what Holmes does, for it is a type of logical inference from guessing. A Sherlock Holmes story usually begins with a display of Holmes’ amazing abilities. Watson is astonished that Holmes knew that he came from Afghanistan and that the man walking out in the street was a retired military man. He is even more astonished at Holmes' actions at the crime scene; the latter comes up with a portrait of the killer and enlightens the Scotland Yard detectives on several important components of the case. At the end of the novel Holmes refers to his skills as reasoning backwards, not forwards.
The ineffectiveness of the police
While Holmes admits that he does indeed respect Lestrade and Gregson as the best of the Scotland Yard detectives, overall the official law enforcement does not seem particularly effective. This theme is present in many other works in the Sherlock Holmes canon but gets its start in the first novel, A Study in Scarlet. Lestrade and Gregson make precipitous conclusions and erroneous assumptions. In addition, they arrive with preconceptions, finger the wrong man for the crime, and miss the nuances of the crime scene. They are often disdainful and jealous of Holmes; their own pride sometimes gets in the way of their acknowledgment that Holmes is actually far better at their jobs than they are. The constable John Rance, who was the one who discovered Drebber's body in the house, completely misses the fact that the drunk man in the street was Jefferson Hope, the murderer. Holmes has no tolerance for his limited abilities. Thus, this novel demonstrates that Holmes's keen intellect and unique abilities exalt him far above the regular police force.
The dangers of organized religion
Arthur Conan Doyle was raised in a Catholic family but eventually chose to leave the faith. Later in life he immersed himself in Spiritualism. He was never a fan of organized religion, which is quite conspicuous in this work. The Mormons are terrifying villains. Their leader Brigham Young is young and fiery, possessed of arrogance and imperiousness. The Danite Band, or the Avenging Angels, terrorize anyone who dissents from the creed. The faith is characterized by secretiveness, oppression, corruption, bribery, hypocrisy, and violence. Anyone who is perceived as a blasphemer faces death or mysterious disappearance. There are rumors of murdered immigrants and kidnapped women. Doyle is suggesting that organized religion squashes independence, autonomy, and freedom of thought. Those who were persecuted can easily turn to persecuting those who they believe are threatening them.
Revenge
Sherlock Holmes eventually comes to the conclusion that it is either politics or love that motivates the murderer of Drebber and Stangerson. Indeed, Jefferson Hope is motivated by love, but a more apposite way of characterizing his motivation is revenge. As a solitary, reticent, strong, and independent man, he does not take well to the fates of Lucy and John Ferrier. He decides that he will devote his life to bringing their killers to justice. This passion for revenge motivates him for decades. Nothing, including a lack of money or information concerning the whereabouts of his chosen victims or having to travel all over Europe in pursuit, will deter him. He refers to himself as the judge, jury, and executioner. He has no faith that God or the law will be able to hold the men accountable for their actions. This desire for revenge is near primal in nature; it is utterly consuming and more than likely directly results in Hope’s deadly aortic aneurism.
Modernism
The Sherlock Holmes stories take place in a historical era characterized by modernity, rationalization, bureaucracy, and scientific progress. Darwin's theory of evolution had recently presented itself as a major threat to the dominance of religious logic. The Industrial Revolution had forever changed the ways in which Britons lived and worked. A population boom and the concomitant disparity between rich and poor were salient aspects of the time. Developments in technology filtered down into the art of criminal science, which is where Holmes establishes his predominance. He is first seen developing a new method for testing blood and is often within the laboratory working on other experiments. His method for solving crimes is based on reason and rationality. He uses his eyes and his intellect to draw evidence from what he observes. His method is orderly and pragmatic, thus emphasizing the way in which deductive reasoning was seen as the most effective way in which to make decisions and judgments in society at large.
Forensic science
Sherlock Holmes dabbles in forensic science before it was even a widespread discipline. He is often discussed as an important influence for actual criminal scientists. In this novel he is first revealed to Watson and the reader while hard at work in his laboratory on a blood test. He speaks of testing a drop of water to note its origins. He looks at footprints and tobacco ash to make conclusions. He sniffs the deceased's mouth for poison. In other works he uses a magnifying glass and a microscope. He looks at documents and tests ballistics. The novels elevate the art of solving crimes to a new high with their emphasis on science as a way in which to solve the crime and ascertain the identity of the criminal.
Friendship
Sherlock Holmes and Dr. John Watson have one of the most famed friendships in literature. This novel depicts their initial meeting and lays the groundwork for their continued association throughout the dozen of Sherlock Holmes tales. Their relationship is successful due to mutual respect and intellectual compatibility, as well as complementary personalities. Holmes is eccentric, Watson stable and restrained. Holmes is volatile, Watson calm. Holmes wants an audience, Watson prefers to study mankind. Their vices and flaws do not interfere with their easy life together. They also share an interest in music. Watson is an admiring and able associate for Holmes's investigations, serving as an effective chronicler. Holmes is not one to have many friends due to his eccentricities and Watson is virtually alone in London. Their decision to take rooms together initiates their celebrated friendship.
