In every case the story opens with a perfect state of Yin. Faust is perfect in knowledge; Job is perfect in goodness and prosperity; Adam and Eve are perfect in innocence and ease; the Virgins—Gretchen, Danae and the rest—are perfect in purity and beauty. In the astronomer’s universe the Sun, a perfect orb, travels on its course intact and whole. When Yin is thus complete, it is ready to pass over into Yang. But what is to make it pass? A change in a state which, by definition, is perfect after its kind can only be started by an impulse or motive which comes from outside. If we think of the state as one of physical equilibrium, we must bring in another star. If we think of it as one of psychic beatitude or nirvana, we must bring another actor on to the stage: a critic to set the mind thinking again by suggesting doubts; an adversary to set the heart feeling again by instilling distress or discontent or fear or antipathy. This is the role of the Serpent in Genesis, of Satan in the Book of Job, of Mephistopheles in Faust, of Loki in the Scandinavian mythology, of the Divine Lovers in the Virgin myths.
Arnold J. Toynbee, A Study of History (via hierarchical-aestheticism)