明月不知沟渠心
[míng yuè bù zhī gōu qú xīn]
Literally, this means 'the bright moon doesn't know the heart of the sewer'. This poetic phrase implies indifference towards lowly things, as well as the gap and detachment between idealism (like the beauty and purity symbolized by the moon) and reality (which can be messy like a ditch).