Sherlock Holmes makes a good literary detective in part because he is far from perfect. When The Sign of the Four begins, he is injecting himself with a seven-percent solution of cocaine, and he also administers morphine to himself. He says to Watson, "My mind rebels at stagnation." Holmes has the marks of a type of mental health disorder, such as bipolar disorder, that subjects him to periods of productivity and good moods when he is working and to low moods and depression when he is not working. Other detectives, such as Miss Marple in the Agatha Christie mysteries, do not have these types of faults.
For the most part, however, the reader sees very little of Holmes's personal life or inner self except his acute powers of reasoning. At the end of The Sign of Four, Watson marries Mary Morstan (the woman who Holmes and Watson have been helping in the case), and the police inspector, Jones, receives the credit for solving the case. Sherlock Holmes says, "For me . . . there still remains the cocaine-bottle." It seems that most of what he cares about is drugs and solving cases, and he is himself a mystery. He makes a good literary detective because the reader wants to know more about Holmes himself, whose inner motivations and inner life are shrouded in mystery.