The first chapter is the study of the scarlet letter when all impossibilities are excluded. When you rule out all the impossible situations, the rest, no matter how incredible, are all facts. The original sentence is from the fourth mark in the first chapter of The Adventures of Sherlock Holmes. Four Signs is the second mystery novel by British detective novelist Arthur Conan Doyle with Sherlock Holmes as the protagonist, which was published in February 1890. The story of the novel takes place between 1887 and 1888. It tells the story that mary morstan, a British lady, received an anonymous parcel every year after her father disappeared, which originally involved a conspiracy. This novel objectively exposes and reflects the British colonial activities in India.