The boundary between sleep and reality.

Hypnos (Ὕπνος, Latin Somnus) is the god of sleep in Greek myth. He was the son of Nyx, goddess of night, and the twin brother of Thanatos, god of death. In Homer's Iliad there is a scene in which Hera asks Hypnos for help in lulling Zeus to sleep — Hypnos was afraid at first, but agreed once Hera promised to give him one of the Charites (goddesses of grace) as his wife. He could put any god or mortal to sleep, but the one time he lulled Zeus he was nearly banished forever. Hypnos lived in a cave at whose entrance poppies and herbs grew, and where Lethe, the river of forgetting, flowed. His son Morpheus was the god who shaped the forms of dreams. In 1876 the British physician James Braid established the term hypnosis in medicine. Yet he soon realized that this was not simple "sleep" — the hypnotic state was a borderland between consciousness and the unconscious, a condition of being awake and yet as if asleep.

Hojeobjimong (胡蝶之夢) is the most famous parable Zhuangzi left behind. "昔者莊周夢爲胡蝶, 栩栩然胡蝶也. 自喩適志與. 不知周也. 俄然覺, 則蘧蘧然周也. 不知周之夢爲胡蝶與, 胡蝶之夢爲周與 (Once Zhuang Zhou dreamed he was a butterfly, fluttering and content, a butterfly indeed. He knew only his own delight and did not know that he was Zhuang Zhou. Suddenly he awoke and was unmistakably Zhuang Zhou. But he could not tell — had Zhuang Zhou dreamed he was a butterfly, or was the butterfly now dreaming it was Zhuang Zhou?)." With this brief tale Zhuangzi posed one of the deepest questions in Eastern philosophy — is there any guarantee that what we call "reality" is not in fact a dream? Of the waking consciousness and the dreaming consciousness, which is the "real me"? Zhuangzi's answer was: "The two cannot be told apart." This he called wuhua (物化) — "the transformation of things into one another." Human and butterfly, dream and reality, self and other are all merely transformations arising within a single Dao (道).