It was the famed psychoanalyst Sigmund Freud who described a variety of different types and examples of Freudian slips in his 1901 book The Psychopathology of Everyday Life. "Almost invariably I discover a disturbing influence from something outside of the intended speech," he wrote. "The disturbing element is a single unconscious thought, which comes to light through the special blunder." According to Freud, these errors reveal unconscious thoughts, beliefs, or wishes.