Yin and Yang are two of the basic terms of the traditional Chinese philosophy. The two terms are also used in the theory of the traditional Chinese medical science. Ancient Chinese often explained many things and phenomena in the world by the concepts of Yin and Yang.
Yin and Yang can show abstract attributes, and they also can denote material things. So they include materialist dialectics, and they surpass materialist dialectics which is full of abstract concepts.
About abstract attributes, Yin and Yang are contrary as negative and positive. In the traditional Chinese philosophy, all things which are related can be defined to Yin or Yang in the world. For example, the sun belongs to Yang, and the moon belongs to Yin; the water belongs to Yin, and the fire belongs to Yang; the women belong to Yin, and the men belong to Yang. So we can summarize the method of defining Yin and Yang. The things which are cold, dull, down, declining, inhibiting or condensed belong to Yin. The things which are warm, bright, up, rising, stimulant or evaporating belong to Yang. The north of a mountain and the south of a river belong to Yin. The south of a mountain and the north of a river belong to Yang. If we pay attention to a fish pond in a sunny day of winter, we can see fishes gathered in the north of the pond. Why? They can enjoy the sunshine.
When Yin and Yang denote material things, they belong to Qi (气). Qi is a kind of extremely small substance, and it forms all things of the world. In ancient Greece, the atomism of Democritus said that the basic components of the entire universe were simple, minute, indivisible, and indestructible particles. The theory of Qi is similar to the atomism, but Qi has rich connotations. Qi includes Yang-Qi (阳气) and Yin-Qi (阴气). Yang-Qi and Yin-Qi are often called Yang and Yin for short. A body has Yang-Qi and Yin-Qi. If the two matters are unbalanced, the person feels sick. If Yang-Qi is excessive, the person is hyperpyretic, his tongue fur is yellow, his urine is yellow, and his feces are dry. If Yin-Qi is deficient, the person has low fever and night sweat, and his tongue fur is less. If Yin-Qi is excessive, the person feels cold, his tongue fur is white and thick, his feces are watery, and his abdomen has cold pain but he refuses to press it. If Yang-Qi is deficient, the person feels cold, he is languid and lethargic, he is willing to press his abdomen, and his urine is more and colorless. The basic principle of cure is to strengthen the deficient and weaken the excessive. The principle of strengthening the deficient is to reinforce Yang-Qi or Yin-Qi, and the principle of weakening the excessive is to use the medicine of cold attribute to clear heat away or use the medicine of hot attribute to warm up.
The attributes of Yin and Yang are not absolute. We can say Qi belongs to Yang, and blood belongs to Yin, and we can say Yang-Qi and Yin-Blood. Why? Qi and blood are two different kinds of substances, but they can’t be said to be opposite. However, Qi is evaporating and blood is condensed, so we can say Yang-Qi and Yin-Blood (阴血).
Yin and Yang move all the while. For example, the Qi of nature includes Yang-Qi and Yin-Qi. In the daytime, Yang-Qi is dominant. In the nighttime, Yin-Qi is dominant. In the noon, Yang-Qi is the strongest, but after the noon Yang-Qi declines gradually, and Yin-Qi rises gradually. In the midnight, Yin-Qi is the strongest, but after the midnight, Yin-Qi declines gradually, and Yang-Qi rises gradually. Why do persons who are critically ill always die at late-night time? It is because at that time Yang-Qi which can strengthen and promote the vitality of a human is very weak.
The contents of the theory of Yin and Yang are very rich, and this article only refers to some basic contents, but it is helpful for a Westerner to understand Yin and Yang which belong to the important thoughts of Oriental people.