How many times have you been told not to judge a book by its cover? Well, Sherlock Holmes would say – no, totally, feel free to judge a book by its cover. Or a woman by her clothing (see Mary Sutherland in "A Case of Identity"). Or a man by his compass ornament and fish tattoo (see Jabez Wilson in "The Red-Headed League"). Borrowing from the example of Edgar Allan Poe's detective Dupin, Holmes spends a lot of time figuring out the character traits of the people around him based on their physical appearance. Also see our section on "Characterization" for more examples of this theme.
When Holmes is fooled by appearances, he seems less omniscient (read: "all-knowing") and more sympathetic to the reader.
Holmes uses categories like class, gender, and even race to make immediate assessments about a person's inner character.