Not two substances, and not opposed forces: one process described from two sides. Day and night, winter and summer, rising and settling are yin-yang pairs, and health is the pair keeping its alternation rather than one side winning.
The pair begins as topography: 陰 is the shaded side of a hill, 陽 the sunny side. Everything else in the corpus is an extension of that—not a metaphor added on top, but the same notion applied to time, temperature, and the body’s own rhythm.
That is why the chapter says the people who knew the Way took yin and yang as their pattern (法於陰陽). To take them as a pattern is to keep time: rising and resting with the season rather than against it.
Passages where the term does real work, with the original, pinyin and English on the same line.
Every rendering below is a decision about which reader is being addressed. There is no neutral option.
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Reading path
This term belongs to Foundations — the five core concepts, in the order the Suwen introduces them.