Sigmund Freud's The Interpretation of Dreams, published in 1899, is usually read as launching an entirely new theory of the unconscious, but its more precise historical achievement was breaking the two-thousand-year-old method the practice had used until then, the lookup key exemplified by Artemidorus in the second century, where a fixed dictionary of symbols was applied to a dreamer's image once the interpreter knew the dreamer's station in life.
Freud proposed instead that a dream's meaning could not be read off a shared symbol table at all, since the same image was disguised differently by each dreamer's own individual censorship of forbidden wishes, and that the only route to its meaning ran through the dreamer's own free associations to the images, a method he called the royal road to the unconscious and that made the dreamer, rather than the interpreter's reference book, the necessary source of the interpretation. This was a genuine rupture rather than a refinement: Artemidorus and the ancient dream books this practice descends from, along with the rabbinic and Islamic traditions that separately classified dreams as true, false or confused, all assumed an interpreter possessed of external, transmissible knowledge that could be applied to any qualifying dreamer, whereas Freud's method could not be looked up in advance for any dream at all, since it depended entirely on material only that dreamer could supply. Later depth psychologists, Carl Jung foremost, kept Freud's turn toward the dreamer's own material while rejecting his more sexually reductive readings, but the essential break, from a shared key to a personal excavation, has held across every major psychological school that has practised dream interpretation since.