Yin - Yang
Early Chinese thought developed the concept that there are two fundamental properties of the universe: yin and yang. These polar opposites are perceived to be present in all things and interconnected by ch'i.
Yin and yang represent a way of thinking in which all things are but a part of a whole. No thing can exist in and of itself but can only be defined by its opposite. For instance, hot is the absence of cold, dark is the absence of light, wet is the absence of dry and so on. Yin and yang are thus opposite properties that define each other.
Yin is associated with the qualities of cold, dark, matter, passivity and rest, and considered feminine. Yang is associated with the qualities of heat, light, energy, activity and movement, and considered masculine.
In yin-yang theory, these two properties are in constant interaction and relation. All things have two aspects, a yin aspect and a yang aspect, and yin and yang mutually create each other and control or balance each other.
The famous tai qi or yin yang symbol, represents the relation and interdependence between yin and yang. The white dot in the black and the black dot in the white signify that in yin (black) there is always the seed of yang (the white dot), and vice versa. No thing is, or can be, all yin or all yang. Yin and yang are complementary properties that compose the whole, and the curved line between them expresses the movement of yin transforming into yang and yang transforming into yin. The fact that the black and white sides are equal demonstrates the balance of yin and yang when there is harmony.
It could be said that the whole of Chinese medicine - its physiology, its pathology, its diagnosis and its treatment - can all be reduced to the fundamental theory of yin and yang. This theory serves to explain the organic structure, physical function and pathological changes in the human body, and in addition guides the clinical diagnoses and treatment. Ch'i flowing through the meridians and following the Law of Five Elements is the mechanism by which yin and yang are expressed within the body.
Ref: C Krebs & T O’Neill McGowan 2014, Energetic Kinesiology