In 1949 Joseph Campbell (1904-1987) made a big splash in the field of mythology with his book The Hero With a Thousand Faces. This book built on the pioneering work of German anthropologist Adolph Bastian (1826-1905), who first proposed the idea that myths from all over the world seem to be built from the same "elementary ideas." Swiss psychiatrist Carl Jung (1875-1961) named these elementary ideas "archetypes," which he believed to be the building blocks not only of the unconscious mind, but of a collective unconscious. In other words, Jung believed that everyone in the world is born with the same basic subconscious model of what a "hero" is, or a "mentor" or a "quest," and that's why people who don't even speak the same language can enjoy the same stories.
Jung developed his idea of archetypes mostly as a way of finding meaning within the dreams and visions of the mentally ill: if a person believes they are being followed by a giant apple pie, it's difficult to make sense of how to help them. But if the giant apple pie can be understood to represent that person's shadow, the embodiment of all their fears, then the psychotherapist can help guide them through that fear, just as Yoda guided Luke on Dagoba. If you think of a person as a computer and our bodies as "hardware," language and culture seem to be the "software." Deeper still, and apparently common to all homo sapians, is a sort of built-in "operating system" which interprets the world by sorting people, places, things and experiences into archetypes.
Campbell's contribution was to take this idea of archetypes and use it to map out the common underlying structure behind religion and myth. He proposed this idea in The Hero With a Thousand Faces, which provides examples from cultures throughout history and all over the world. Campbell eloquently demonstrates that all stories are expressions of the same story-pattern, which he named the "Hero's Journey," or the "monomyth." This sounds like a simple idea, but it suggests an incredible ramification, which Campbell summed up with his adage "All religions are true, but none are literal." That is, he concluded that all religions are really containers for the same essential truth, and the trick is to avoid mistaking the wrappings for the diamond.
Lucas had already written two drafts of Star Wars when he rediscovered Joseph Campbell's The Hero With a Thousand Faces in 1975 (having read it years before in college). This blueprint for "The Hero's Journey" gave Lucas the focus he needed to draw his sprawling imaginary universe into a single story.
Note that the Wachowski Brothers' wonderful film The Matrix is carefully built on the same blueprint:
The call to adventure
Princess Leia's message
"Follow the white rabbit"
Refusal of the call
Must help with the harvest
Neo won't climb out window
Obi-wan rescues Luke from sandpeople
Trinity extracts the "bug" from Neo
Crossing the first threshold
Neo is taken out of the Matrix for the first time
The belly of the whale
The road of trials
Sparring with Morpheus
The meeting with the goddess
Princess Leia (wears white, in earlier scripts was a "sister" of a mystic order)
Temptation away from the true path1
Luke is tempted by the Dark Side
Cypher (the failed messiah) is tempted by the world of comfortable illusions
Atonement with the Father
Darth and Luke reconcile
Neo rescues and comes to agree (that he's The One) with his father-figure, Morpheus
Apotheosis (becoming god-like)
Luke becomes a Jedi
Neo becomes The One
The ultimate boon
Death Star destroyed
Humanity's salvation now within reach
Refusal of the return
"Luke, come on!" Luke wants to stay to avenge Obi-Wan
Neo fights agent instead of running
The magic flight
Rescue from without
Han saves Luke from Darth
Trinity saves Neo from agents
Crossing the return threshold
Millennium Falcon destroys pursuing TIE fighters
Neo fights Agent Smith
Master of the two worlds
Neo's declares victory over machines in final phone call
Freedom to live
Rebellion is victorious over Empire
Humans are victorious over machines
Common Mythic Elements
Two Worlds (mundane and special)
Planetside vs. The Death Star
Reality vs. The Matrix
Luke will overthrow the Emperor
Morpheus will find (and Trinity will fall for) "The One"
In an early version of the script, Morpheus once believed that Cypher was "The One"
Wearing Enemy's Skin
Luke and Han wear stormtrooper outfits
Neo jumps into agent's skin
Shapeshifter (the Hero isn't sure if he can trust this character)
Chasing a lone animal into the enchanted wood (the animal usually gets away)
Luke follows R2 into the Jundland Wastes; The Millennium Falcon follows a lone TIE fighter into range of the Death Star
Neo "follows the white rabbit" to the nightclub where he meets Trinity
Searching For The Hero
Jung and Campbell aren't the only people who have attempted to map the Hero's Journey! There have been dozens of less celebrated forays into this area, and just about everyone comes to slightly different conclusions. Notable works include:
German philologist Max Müller (1823-1900) got the ball rolling with an influential essay called Comparative Mythology, first published in Oxford Essays (1856). According to The Columbia Encyclopedia, "Müller did more than any other scholar to popularize philology and mythology, particularly in his lectures Science of Language (1861, 1863)." In my opinion Müller's scholarship was weakened because he proceeded from two basic assumptions without questioning them: (1) that fairytales were inferior drivel for children, and (2) that his faith was valid and all other faiths were primitive superstitions. For instance, Müller was one of the first people to introduce Hindu scripture into Europe, by translating the Rig-Veda. It was of course unthinkable that the Veda could actually be older than The Hebrew Bible, and anyway the King James Bible says right there in black and white (in a margin note new to the 1611 edition) that the Earth was created in 4,004 BCE, so Müller changed all the Vedic dates to suit that "historical fact." Eeek! (It turns out parts of the Veda seem to have been composed before 4,004 BCE. So the Vedic faith began before the King James version of the Earth even existed. Müller also wrote disparaging essays about German folktales, which tend to revolve around an idea called poetic irony.) Müller wrote that the Vedas are mostly "childish in the extreme, tedious, low and commonplace." Can a translator really do justice to a work he holds in contempt? Müller's two-volume Contributions to the Science of Mythology (1897) argued that mythology is "a disease of language." J.R.R. Tolkien, in his 1938 essay On Fairy-Stories, countered "It would be more near the truth to say that languages, especially modern European languages, are a disease of mythology." In all fairness it's a bit petty to make fun of Müller for his religious intolerance, particularly since the other 19th-century German academics he hung out with considered him bizarrely respectful of the "silly superstitious beliefs" of non-Christians. Müller was a brilliant man who laid the foundations for modern Linguistics and Mythology, and he's still worth reading.
Englishman Sir Edward Burnett Tylor (1832-1917), sometimes called the "father of cultural studies," wrote a book called Primitive Culture: Researches into the Development of Mythology, Philosophy, Religion, Language, Art and Custom (1871). His idea was to demonstrate that all religion began with animism (the belief that everything has a soul), followed by polytheism, followed by monotheism. Tylor's book became a bit of a scandal because it provided scientific support for the idea that "primitive" peoples weren't inferior to white Europeans. His work was partially a counter-argument to Müller.
Austrian scholar Johann Georg von Hahn (1811-1869) mapped out a few common elements he found in the Greek and Albanian fairytales he had translated, in his book Sagwissenschaftliche Studien (roughly "Fairytale Sudies," 1876).
Alfred Nutt (1856-1910) observed a few underlying commonalities of myth from his studies of the Celts, in his The Aryan Expulsion-and-Return Formula in the Folk and Hero Tales of the Celts (1881). He also wrote The Fairy Mythology of Shakespeare (1900).
Viennese psychoanalyst Otto Rank (1884-1939) created an influential Freudian interpretation of the Hero's Journey by analyzing 34 European and Near-Eastern hero stories, in his book Myth of the Birth of the Hero (1909). As you might guess from the title, Rank didn't think much of myths and fairytales.
The Golden Bough: A Study in Magic and Religion (1911-1915), by Scottish anthropologist Sir James George Frazer (1854-1941). Draws parallels between mythic beliefs of cultures from all over the world. Frazer's book is outdated in a few small ways, but is still considered one of the great books of the twentieth century, and for excellent reason: Frazer was the first person in the world to map out the mechanics of what we might call "magical thinking", the essential building block of mythos ("sacred story," Greek). The book makes an excellent argument that magical thinking is one of the primary keys to understanding human consciousness. Frazer was an enormous influence on Joseph Campbell, Ursula Le Guin and Frank Herbert. If you nurture ambitions to write, especially to write "mythic" fiction, Frazer's brilliant book is without question on the short-list of recommended reading.
Russian professor Vladimir Propp (1895-1970) published an extremely influential analysis of Russian folktales called Morfologiia Skazki ("Morphology of the Folktale", 1928), which identified 31 steps on the hero's journey.
Lord Fitzroy Richard Somerset Raglan (1885-1964) wrote a very influential book called The Hero (1934), in which he identified 22 steps on the Hero's Journey. His work drew heavily on Frazer's Golden Bough.
Dutch folklorist Jan de Vries identified ten common elements in the Hero's Journey, in his Forschungsgeschichte der Mythologie (1961, published in English as "Heroic Song and Heroic Legend" 1963). His ideas seem to be strongly influenced by von Hahn's work.
French anthropologist Claude Lévi-Strauss (1908-) was one of the chief proponents of structuralism (the academic vogue which immediately preceded postmodernism). His Mythologiques is based on an exhaustive analysis of Native American myths, written between 1964-1971. Lévi-Strauss's efforts to isolate the "atomic elements" of myth are well-considered and interesting, though his writing style is extremely dense, self-referential and oblique, so you might need to read this book several times to understand what he's trying to communicate. I can't say I fully understand his points, but there are many fans of this book who report that all the hard work it requires is worthwhile.
1 In 1949 Campbell called this mythic element "woman as temptress," but both Lucas and the Wachowski Brothers (as well as countless others) have managed to imagine compelling temptations away from the Divine Path which didn't cast woman as the villain. Campbell's idea that woman represents divine impurity was probably influenced by his orthodox Catholic upbringing, which shifts the responsibility for Adam's fall to Eve. Although Campbell's book is scrupulously cross-cultural, the one exception is his chapter on "Woman As the Temptress," which draws examples exclusively from Judeo-Christian myths.
Sources include: The Hero With a Thousand Faces, by Joseph Campbell (1949); The Hero's Journey - Joseph Campbell on his Life and Works (1990)