“We always know when we are awake that we cannot be dreaming even though when actually dreaming we feel all this may be real.”

Source: The Butterfly as Companion: Meditations on the First Three Chapters of the Chuang-Tzu
Context: How do I know that enjoying life is not a delusion? How do I know that in hating death we are not like people who got lost in early childhood and do not know the way home? Lady Li was the child of a border guard in Ai. When first captured by the state of Jin, she wept so much her clothes were soaked. But after she entered the palace, shared the king's bed, and dined on the finest meats, she regretted her tears. How do I know that the dead do not regret their previous longing for life? One who dreams of drinking wine may in the morning weep; one who dreams weeping may in the morning go out to hunt. During our dreams we do not know we are dreaming. We may even dream of interpreting a dream. Only on waking do we know it was a dream. Only after the great awakening will we realize that this is the great dream. And yet fools think they are awake, presuming to know that they are rulers or herdsmen. How dense! You and Confucius are both dreaming, and I who say you are a dream am also a dream. Such is my tale. It will probably be called preposterous, but after ten thousand generations there may be a great sage who will be able to explain it, a trivial interval equivalent to the passage from morning to night.

Social Dreaming of the Frin in David G. Hartwell (ed.) Year's Best Fantasy 3, p. 172 (Originally published at The Magazine of Fantasy & Science Fiction https://en.wikipedia.org/wiki/The_Magazine_of_Fantasy_%26_Science_Fiction October/November 2002)

Source: The Angel's Game

Variants:
Novalis (1829)
Variant: We are near awakening when we dream that we dream.
Source: Novalis: Philosophical Writings

"Más exclusivamente que en la vigilia, en el sueño somos nosotros. Contribuimos con todo el reparto."
Descanso de caminantes, 2001.

Source: A Midsummer Night's Dream

1983