“Literature and butterflies are the two sweetest passions known to man.”

The Functions of Criticism at the Present Time (1864)

Source: The Magic Mountain (1924), Ch. 7

As translated by Lin Yutang
Alternative translations
Once upon a time, I, Chuang Chou, dreamt I was a butterfly, fluttering hither and thither, a veritable butterfly, enjoying itself to the full of its bent, and not knowing it was Chuang Chou. Suddenly I awoke, and came to myself, the veritable Chuang Chou. Now I do not know whether it was then I dreamt I was a butterfly, or whether I am now a butterfly dreaming I am a man. Between me and the butterfly there must be a difference. This is an instance of transformation.
As translated by James Legge, and quoted in The Three Religions of China: Lectures Delivered at Oxford (1913) by William Edward Soothill, p. 75
Once Zhuang Zhou dreamed he was a butterfly, a fluttering butterfly. What fun he had, doing as he pleased! He did not know he was Zhou. Suddenly he woke up and found himself to be Zhou. He did not know whether Zhou had dreamed he was a butterfly or a butterfly had dreamed he was Zhou. Between Zhou and the butterfly there must be some distinction. This is what is meant by the transformation of things.
One night, Zhuangzi dreamed of being a butterfly — a happy butterfly, showing off and doing things as he pleased, unaware of being Zhuangzi. Suddenly he awoke, drowsily, Zhuangzi again. And he could not tell whether it was Zhuangzi who had dreamt the butterfly or the butterfly dreaming Zhuangzi. But there must be some difference between them! This is called 'the transformation of things'.
Once upon a time, Chuang Chou dreamed that he was a butterfly, a butterfly flitting about happily enjoying himself. He didn’t know that he was Chou. Suddenly he awoke and was palpably Chou. He didn’t know whether he were Chou who had dreamed of being a butterfly, or a butterfly who was dreaming that he was Chou.
Context: Once upon a time, I, Chuang Chou, dreamt I was a butterfly, fluttering hither and thither, to all intents and purposes a butterfly. I was conscious only of my happiness as a butterfly, unaware that I was Chou. Soon I awaked, and there I was, veritably myself again. Now I do not know whether I was then a man dreaming I was a butterfly, or whether I am now a butterfly, dreaming I am a man. Between a man and a butterfly there is necessarily a distinction. The transition is called the transformation of material things.

for better or for worse.
Nobel lecture (2001)

Source: Rosencrantz and Guildenstern Are Dead

Walesa's Wife, from Practicalities (1987, trans. 1990).

Source: Russian Winter