Under heaven all can see beauty as beauty only because there is ugliness.
    All can know good as good only because there is evil.
    
    Therefore having and not having arise together.
    Difficult and easy complement each other.
    Long and short contrast each other:
    High and low rest upon each other;
    Voice and sound harmonize each other;
    Front and back follow one another.
    
    Therefore the sage goes about doing nothing, teaching no-talking.
    The ten thousand things rise and fall without cease,
    Creating, yet not possessing.
    Working, yet not taking credit.
    Work is done, then forgotten.
    Therefore it lasts forever.

-- Tao Te Ching verse 2.