Everything comes in pairs

as death with life and darkness with light. Whatever seems to be necessary to existence is so only because its opposite is equally necessary. Duality is a governing factor of the world and everything within it including ourselves. That alone is outside the world, is nondual, which is the untouchable Reality. This is the Chinese idea of yin and yang, and the Bhagavad Gita's expression "the pairs of opposites" conveys the same idea. Duality is a fact. It is here. But it is also an illusion and the opposite truth which completes it is the nondual. We may deplore the illusory nature of our existence, but we need not get lost in it for it is fulfilled, completed, and finalized in its complement the Real.
  
--Paul Brunton, Notebooks
Nondual Highlights 2840