The concept of yin and yang describes two opposing and, at the same time, complementary aspects of any one phenomenon or comparison of any two phenomena. They are universal standards of quality at the basis of the systems of correspondence seen in most branches of classical Chinese science and philosophy, traditional Chinese medicine being an example.
''Yin'' qualities are characterized as soft, slow, substantial, wet, cold, conserving, tranquil, gentle, and corresponding to the night.
''Yang'' qualities are characterized as hard, fast, insubstantial, dry, hot, excited, aggressive, and corresponding to the day.
Four Laws of Yin-yang
* Yin-yang are Opposing
** Yin and yang describe the polar effects of phenomena. In viewing any one phenomenon , yin and yang describe the opposing qualities inherent in it. For instance, winter and summer would be the yin and yang, respectively, of the year.
* Yin-yang are Mutually Rooted
** Yin and yang are two complementary qualities. That is to say, the yin and the yang aspect of any one phenomenon will, when put together, form the entire phenomenon. Yin-yang is a philosophy of duality. This is the reason the Chinese word has no "and" between yin and yang - the term always expresses the two making up the one. In the example above, winter plus summer makes up the whole year.
* Yin-yang Mutually Transform
** The maximum effect of one quality will be followed by the transition toward the opposing quality. In other words, once the maximum Yang aspect has manifest, such as the long days of summer, this will be followed by the transition toward the Yin aspect, with the shortening of the days as winter approaches.
* Yin-yang Mutually Wax and Wane
** The Yin and yang aspects are in dynamic equilibrium. As one aspect declines, the other increases to an equal degree. For instance, in the cycle of the year, the long days of summer gradually shorten and the nights gradually lengthen as winter approaches. Throughout the process, however, the length of each day is constant while it is only the relative length of light and darkness that changes .
Yin and yang are neither substances nor forces. They are the terms used in a system of dualistic qualification which can be applied universally. Further dividing Yang and Yin into their respective Yin and Yang aspects, yields four combinations: the Yin of the Yang, the Yang of the Yang, the Yin of the Yin, and the Yang of the Yin. This allows an endless scale of universally defined qualities, which is foundational to classical Chinese thought, as seen in the Tao Te Ching, and science, as seen in the Yellow Emperor's Huangdi Neijing.
The Taijitu or Taiji Diagram , often referred to as ''yin-yang'' in , is a well known symbol deriving from Chinese culture which represents the principle of yin and yang from Taoist and Neo-Confucian philosophy. The term ''Taijitu'' itself refers to any of several schematic diagrams representing these principles.
The taijitu represents an ancient Chinese understanding of how things work. The outer circle represents the entirety of perceivable phenomena, while the black and white shapes within the circle represent the interaction of two principles or aspects, called "yin" and "yang" , which cause the phenomena to appear in their peculiar way. Each of them contains an element or seed of the other, and they cannot exist without each other. There are other ways that Chinese schools of thought graphically represented the principles of yin and yang, an older example being the solid and divided lines of the ''I Ching''.
Wu Jianquan, a famous Chinese martial arts teacher, described the name of the martial art Taijiquan this way at the beginning of the 20th century:
:"Various people have offered different explanations for the name ''Taijiquan''. Some have said: - 'In terms of , one must train from a state of movement towards a state of stillness. ''Taiji'' comes about through the balance of ''yin'' and ''yang''. In terms of the art of attack and defense then, in the context of the of full and empty, one is constantly internally latent, not outwardly expressive, as if the ''yin'' and ''yang'' of ''Taiji'' have not yet divided apart.' Others say: 'Every movement of ''Taijiquan'' is based on circles, just like the shape of a ''Taijitu''. Therefore, it is called ''Taijiquan''.' Both explanations are quite reasonable, especially the second, which is more complete."
In the image showing yin-yang as a circle the white part represents yang and the black part represents yin. Two parts pass through each other on a line because yin and yang are never separated. There is a small black round in white part and a white one in the black part.
In yin yang cosmology, the taiji is preceded by . Wuji separates into taiji or yin and yang . Two symbols became four symbols. Subsequently, four symbols became . And at last, bagua describes the myriad things of creation. This framework underlies both the I Ching and Tao Te Ching.
Other uses of Taijitu-similar symbols
The Yin and Yang symbol was also used on the shields of late military units as regional identifications, as evident from the Notitia Dignitatum. The '''' "Armigeri propugnatores seniores", and the '''' "Mauri Osismiaci" used a variation of the Taijitu symbol on their shields. Note the symbol was not necessarily adopted because of any Chinese influence, and may have been a native European symbol as well.
Dichotomy in other philosophies
The concept of "unity in duality" as underlying the nature of the Cosmos is fundamental in the philosophy of Heraclitus, one of the Presocratics. Note that the Heracletian duality has nothing to do with an dualism.
Indian philosophy has a distinct dualistic strand, known as Samkhya. In this theory, Yin corresponds to Prakrti and Yang to Purusha.
Gnosticism and Zoroastrianism posit a supernatural dualism to explain suffering in this world.
Indonesia has the motto: "Bhinneka Tunggal Ika" that is "Unity in diversity" which originates from a quotation of an Old Javanese poem. This idea is similar to yin and yang philosophy. "Tunggal" means the only one, as in "anak tunggal" .
Taijitu is defined in code point U+262F . As an alternative, Unicode suggested it can be substituted by U+0FCA , the double body symbol .