In the second part of the story "A Study in Scarlet", Sherlock Holmes's method is explained in detail. He is the only Consulting Detective in the world and his "science of detection" is based on analysis and deduction, figuring things out from things he already knows. He is especially good at backwards reasoning (inference or abduction). From the scratches around on an old watch, he decides the man drinks a lot; in another case seeing the man's clothes are respectable but worn, he thinks the man was once successful, but has fallen on hard times and taken to drink.