Yin and yang was first conceived through patient observation of the forces of nature. The Taoists who developed the system of traditional Chinese medicine saw the universe as a unified field, constantly moving and changing while maintaining its oneness.
This constant state of change was explained through the theory of yin and yang, which appeared in written form around 700 B.C. in the I Ching ("Book of Changes").
According to the theory, nature expresses itself in an endless cycle of polar opposites such as day and night, moisture and dryness, heat and cold, and activity and rest.
Yin phenomena are those that exhibit the nurturing qualities of darkness, rest, moisture, cold, and structure. Its Chinese character depicts the shady side of a hill.
Yang phenomena have qualities of energy such as light, activity, dryness, heat, and function. Its Chinese character represents the sunny side of a hill.
Everything in nature exhibits varying combinations of both yin and yang. For example, the morning fog (yin) is dissipated by the heat of the sun (yang); the forest fire (yang) is extinguished by the rainstorm (yin); the darkness of night (yin) is replaced by the light of day (yang). Any phenomenon within nature can be understood in relation to another; one will always be yin or yang in comparison with the other.