Two sides of one hill
The characters for yin and yang are old, and their earliest everyday meaning is probably quite concrete. Yin is linked to shade, the shaded side of a hill or the north bank of a river, while yang is linked to sunlight, the sunny side of a hill or the south bank of a river. From these simple observations of light and shade the words grew into a pair of general categories.
Over time, yin came to cover things such as darkness, cold, softness, stillness, receptivity, the moon and water, and yang came to cover brightness, warmth, firmness, movement, activity, the sun and fire. The lists are conventional, not exact: what counts as yin or yang depends on the comparison. A hot day is yang compared with a cold one, yet a summer evening is yin compared with noon.
Interdependent, not opposed
The most important feature of the idea is that yin and yang are not good and evil, and not enemies. They are complementary, and each depends on the other for meaning. Day has no sense without night, and an action has no meaning without rest. They also contain each other in small measure, and shift into each other: when yang reaches its peak at midsummer, yin begins to grow, and the reverse happens at midwinter.
Many modern readers know the circular symbol with a dot of each colour inside the other half. That diagram, the taijitu, is a later development, associated with scholars of the Song period and after, and is not an ancient emblem. The ideas it illustrates are much older, though the visual image should not be read back into early China.
Early sources
The idea does not belong to a single author or school. It appears in several streams of thought:
- The Book of Changes (Yijing). The core of this divination text uses sixty-four figures built from lines that are either solid or broken. Later commentaries, grouped as the “Ten Wings” and composed over several centuries, interpret the solid and broken lines through yin and yang, and describe change as the interaction of the two. The Book of Changes is thus central, though the exact stages of its formation are debated.
- The Tao Te Ching. A well-known chapter says that the ten thousand things carry yin on their backs and embrace yang in front, and that harmony comes from the blending of their breath. This ties the pair to the larger idea of the Tao at the heart of Daoist philosophy.
- The yin-yang thinkers. Writers associated with Zou Yan and others in the late Warring States period built yin and yang into a general theory of nature and history, together with the five phases cycle. Our knowledge of them comes largely through later reports.
- Han syntheses. In the Han period, scholars such as Dong Zhongshu combined yin and yang with Confucian ethics and the idea that heaven and human affairs respond to one another.
Nature and the seasons
Yin and yang offered a way of describing the rhythms of nature. The year was seen as a cycle in which yang grows through spring, peaks in summer, then declines, while yin grows through autumn and peaks in winter. Farming, ritual and the calendar all followed this pattern. A sensible ruler, according to calendar texts, scheduled certain activities for the right season, such as sowing, hunting and punishment, to avoid disturbing the balance. Greek writers likewise tied activity to the year in their Greek seasonal calendar, though the Chinese scheme was more systematic in treating the whole cosmos as a matter of balanced change.
Medicine
In the medical classic Huangdi Neijing, a work compiled over a long period and usually dated to the Han or slightly earlier, yin and yang are used to describe the body. The inner organs and the front of the body were classed as more yin, the outer parts and back as more yang, and health was understood as a state of balance between them. Illness could be described as an excess or deficiency of one or the other, for example a pattern of chills and fever being read as a sign of one or the other.
This is a traditional framework that guided diagnosis and the choice of ancient herbal remedies and other treatments in China. It is not the same as modern physiology, and this article does not make claims about its effectiveness. A useful historical comparison is humoral theory in Greek medicine, which also attempted to explain health as a balance of opposing qualities, although the details and vocabulary differ.
Society and its critics
Yin and yang were also applied to human relationships. Ruler and minister, parent and child, and husband and wife were each described in terms of yang and yin, with the first as leading and the second as following. Han thinkers such as Dong Zhongshu used this to support hierarchy, and later writers repeated the association of yin with women and subordinate roles.
This is a point where the idea has been criticised. The original sense of mutual dependence did not necessarily imply that one was worth more, and some modern scholars argue that the hierarchical reading was a particular Han development, not an inevitable consequence of the concept. Others note that the vocabulary was often used to justify existing social arrangements. The record shows both readings in use.
A way of seeing change
Taken as a whole, yin and yang gave early Chinese thinkers a flexible vocabulary for describing change without a creator god or a war between good and evil. They remained influential in medicine, cosmology and art, and also in everyday language. The pair remains one of the most widely recognised ideas from ancient China, which is also why it is so often misunderstood, and why it is worth reading in light of the original sources.