梦里梦不出梦中梦画里画不出画中画
[mèng lĭ mèng bù chū mèng zhōng mèng huà lĭ huà bù chū huà zhōng huà]
Translated as 'A dream inside a dream can't be dreamed within another dream, a painting inside a painting can't be painted in another picture'. A very complex sentence representing intricate thoughts.