Sir Arthur Ignatius Conan Doyle was a Scottish writer and physician, most noted for his fictional stories about the detective Sherlock Holmes, which are generally considered milestones in the field of crime fiction. The story begins in Holmes and Watson’s Baker Street apartment in London. Holmes, who appears to be deeply engrossed in his chemicals and test-tubes, surprises Watson by apparently reading his mind: “So, Watson . . . you do not propose to invest in South African securities?” Watson, astonished by Holmes’s remark, demands an explanation, and Holmes complies, relating an intricate chain of reasoning that begins with the presence of chalk on Watson’s left hand the previous night and concludes with his investment decision.