In Taoist philosophy, dark and light (☯) yin and yang, arrives in the dàodéjīng at Chapter 42. It becomes sensible from an initial quiescence or emptiness (wuji, sometimes symbolized by an empty circle), and continues moving until quiescence is reached again. For instance, dropping a stone in a calm pool of water will simultaneously raise waves and lower troughs between them, and this alternation of high and low points in the water will radiate outward until the movement dissipates and the pool is calm once more. Yin and yang thus are always opposite and equal qualities. Further, whenever one quality reaches its peak, it will naturally begin to transform into the opposite quality: for example, grain that reaches its full height in summer (fully yang) will produce seeds and die back in winter (fully yin) in an endless cycle. It is impossible to talk about yin or yang without some reference of the opposite; since yin and yang are bound together as parts of a mutual whole. A way to illustrate this idea is to postulate the notion of a race with only men or only women; this race would disappear in a single generation. Yet, men and women together create new generations that allow the race they mutually create (and mutually come from) to survive. The interaction of the two gives birth to things. Yin and yang transform each other: like an undertow in the ocean, every advance is complemented by a retreat, and every rise transforms into a fall. Thus, a seed will sprout from the earth and grow upwards towards the sky – an intrinsically yang movement. Then, when it reaches its full potential height, it will fall.
"Laozi". Stanford Encyclopedia of Philosophy by Stanford University. "The Tao Teh King, or the Tao and its Characteristics by Laozi - Project Gutenberg". Gutenberg.org. 2007-12-01.