What is the theme in The Neverending Story by Michael Ende?

The duality of life is another key theme in The Neverending Story, symbolized by the two snakes that make up the Childlike Empress' emblem. The snakes on her magic medallion, AURYN, hold each other's tails in their mouths, thus forming a circle. Bastian does not understand this concept until he sees the two giant snakes guarding the Water of Life. These snakes, one black and one white, represent good and evil as well as all other opposing concepts. The snakes surrounding the Water of Life are so huge that Bastian realizes if they did not hold each other in check, either snake, individually, would destroy all of Fantastica. This realization summarizes the concept of harmonious duality that the author seeks to impart.