The story of Chuang Tzŭ.
"Once Chuang Tzŭ dreamt he was a butterfly, a butterfly flitting and fluttering around, happy with himself and doing as he pleased. He didn't know he was Chuang Tzŭ. Suddenly he woke up and there he was, solid and unmistakable Chuang Tzŭ. But he didn't know if he was Chuang Tzŭ who had dreamt he was a butterfly, or a butterfly dreaming he was Chuang Tzŭ. Between Chuang Tzŭ and a butterfly there must be some distinction! This is called the Transformation of Things."
See the Wikipedia entry on the Butterfly Dream for more.