The opening section of Chapter 2 of the Tao Te Ching focuses on the unified duality of all things. Traditionally, this unified duality is called Taiji, which means "supreme ultimate principle." Taiji consists of yin and yang, which are the generic names given to the opposing (but ultimately complementary) polarities in all phenomena.
Yin and Yang
By contrasting things against their opposites, Lao-Tzu shows that opposite polarities define each other and give each other meaning:
We can only see beauty as beauty because there is ugliness.
We can only see good as good because there is evil.
