The Alchemist, a character who will not be introduced until much later in the narrative, finds a book that retells the myth of Narcissus. The myth of Narcissus traditionally tells how a youth, whose name was Narcissus, loved his own image so much that he spent days looking at his own reflection in a lake. One day, he was so infatuated with himself that he lost his concentration, slipped and drowned in the lake. A flower grew by where he fell and this plant came to be called the narcissus.
The author of the book the Alchemist finds is different, though. It continues by telling how the goddess of the forest went to the lake after Narcissus had died and found it converted into a lake of tears; the lake was weeping for Narcissus. The lake missed Narcissus because, in the reflection of his eyes, the lake could contemplate itself. This version of the myth makes the Alchemist very happy.
This prologue introduces several elements which will come into play later in the narrative. First, the reader can see the attribution of human traits to inanimate objects - in this case, the lake that cries. This both sets the magical tone of the narrative and highlights the interrelation of all things, animate and inanimate, that forms one of the main themes of the book. Second, the story of the Narcissus highlights the question of selfishness, a question which is at the center of The Alchemist. Is it possible to pursue one's own personal desires while living a good life? In the case of Santiago, the answer is a resounding yes - making the image of Narcissus (who become a flower through his selfishness) especially apt.