A favorite quote:
"The shadow is the other side of our psyche, the dark brother of the conscious mind. It is Cain, Caliban. Frankenstein's monster. Mr. Hyde. It is Vergil who guided Dante through hell, Gilgamesh's friend Enkidu, Frodo's enemy Gollum. It is the Doppelganger. It is Mowgli's Grey Brother; the werewolf; the wolf, the bear, the tiger of a thousand folktales; it is the serpent, Lucifer."
-- Ursula LeGuin