"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 compliment each other. Long and short contrast
each other; high and low rest upon each other; Front and
back follow one another."

Lao Tsu in his famous work To Te Ching. This version
translated by Gia-Fu Feng and Jane English.