The school of Yin 陰 and Yang 陽 thought was one of the philosophical schools of thought during the Warring States period 戰國 (5th cent.-221 BCE). This school is often mentioned side by side with the school of the Five Processes or Five Phases (yinyang wuxing jia 陰陽五行家). The Yin and Yang school originated among the ancient astronomers that observed sun, moon and the constellations and also tried to predict the future from the movements of the starry sky. The oldest description of the Yin and Yang school has been made by the Former Han period 前漢 (206 BCE-8 CE) historiographer Sima Tan 司馬談 who said in his essay Lun liujia yaozhi 論六家要旨 that the diviners of the Yin and Yang school tried to enchain and intimidate the stupid masses with their statements (shi ren ju er duo wei 使人拘而多畏). They had nevertheless made, he admitted, important contributions to the understanding of the astronomical phenomena and the annual seasons and the seasonal phenology. The imperial bibliography Yiwenzhi 藝文志 of the official dynastic history Hanshu 漢書 says that the Yin-Yang thinkers originated in astromical and astrological offices that were in mythical times occupied by Xi He 羲和. They observed the sky and created a calendar and so instructed the peasant people when to undertake their agricultural works.
The terms yin and yang originally denoted the northern, shady side of a hill (yin), and the southern, sunny side respectively (yang), or the northern, sunny banks of a river (yang) and the southern, shady banks of it. In the field of astronomy, the term yin meant the new moon, the term yang the full moon, but yin also denoted the moon itself, while yang was the sun. In the field of phenology, yin meant "winter" or the agent that prevails in winter and forces all plants and small animals to hide deep inside the earth. During winter, the agent yang is also hidden inside the roots of the plants, and starts growing in spring. In summer, the agent yang takes over and reaches its zenith with the beginning of autumn. Yin and Yang are subject to regular cycles.
During the Warring States period, there were two different traditions of the Yin and Yang thinkers. The one tradition connected the Yin and Yang cycle with the cycle of the Five Processes (wuxing 五行). They explained the change of the seasons and gave support to the correlated agricultural activities. The representative writings of this tradition are the chapter Yueling 月令 in the Confucian Classic Liji 禮記, and the twelve annals (ji 紀) of the Lüshi chunqiu 呂氏春秋, as well as agricultural books. The second tradition focused on the Five Processes and their meaning for society and history. Their most important representants were Zou Yan 鄒衍 and Zou Shi 鄒奭 with the lost writings Zouzi 鄒子, Zouzi zhongshi 鄒子終始, and Zoushizi 鄒奭子. While Yin and Yang corresponded to moon and sun, the Five Processes were seen as analogous to the five planets Mercury, Venus, Mars, Jupiter and Saturn. The bibliography Yiwenzhi 藝文志 in the history Hanshu lists 68 books of Yin and Yang thinkers. The Qing period 清 (1644-1911) scholar Ma Guohan 馬國翰 has collected all surviving fragments of Zou Yan's writings, which are now included in the collectaneum Yuhan shanfang jiyi shu 玉函山房輯佚書. Quotations of Yin-Yang thinkers are to be found in the books Lüshi chunqiu, Huainanzi 淮南子, Chunqiu fanlu 春秋繁露 and Baihu tongyi 白虎通義.
The theory of Yin and Yang had a great impact on a wide range of traditional Chinese philosophies, from medical theories like explained in the Huangdi neijing 黃帝內經 to the critics of superstition as seen in Wang Chong's 王充 Lunheng 論衡 and their counterpart, the apocryphal Classics (chenwei 讖緯).
Yin and Yang also play a great role in the Confucian Classic Yijing 易經 "Book of Changes", and its explanation by the Neo-Confucians. Yin and Yang are also known as agents in cosmic theories in Daoism.
The theory of the Five Processes was originally a separate tradition. The theory of the Five Processes was very popular during the Han period 漢 (206 BCE-220 CE) but continued to play a certain role in traditional Chinese thinking. It was believed that all things on earth were influenced by a permanent but irregular change of the Five Processes or Phases. A change of the phase would be visible by strange events happening on earth, like the appearance of phoenixes, coloured clouds, the transformation of a cock into a hen, and so on. The change of the phases is described as different cycles, namely the productive cycle (sheng 生) or victorious (sheng 勝) and the destructive cycle (ke 克). By means of prognostication methods, a change could be predicted. Each of the Five Phases was characterized by one element, each of which had an effective force on all things on earth. The Five Elements (wucai 五材) are therefore also called the Five Forces (wude 五德). The earliest statement in literature about the Five Processes can be found in the chapter Hongfan 洪範 of the Confucian Classic Shangshu 尚書, where they are definded as water (shui 水), fire (huo 火), wood (mu 木), metal (jin 金) and earth (tu 土). The Sui period 隋 (581-618) scholar Xiao Ji 肖吉 explains the cycle of the phases replacing each other (xiangke 相克 or xiangsheng 相勝) in his book Wuxing dayi 五行大義: wood overcomes earth, earth overcomes water, water overcomes fire, fire overcomes metal, and metal overcomes wood. This cycle is actually a description of natural processes: An iron hoe works the soil, trees grow out of the soil and transform it into wood, earthen dykes can control the water, water extinguishes fire, and fire melts down iron. The other cycle says that wood produces fire (by burning it), fire produces earth (ashes), earth produces metal (ores), metal produces water (liquid metal), and water produces wood (growing plants).
The Mohist Canon also mentions the theory of the Processes but stresses that the chance of one element to overcome the other (jiaosheng 交勝) depended on the mass or prevalence, so that it was by no means certain that the cycle would move into one predestined direction. On the other side it was the fact that all elements were integrated into each other and nourished each other (xiangli 相麗). Frozen water was integrated in hard metal, while fire was hidden in the fuel wood. Especially in Mohist scientific thinking it can be seen that the theories of the Five Processes could be applied to explain natural phenomena in a time when science was still underdeveloped.
Later on, the theory of the Five Processes was used to explain changes in history. Changes in the phases would express a change of Heaven's favour for one dynasty. The earth, with the colour yellow, was the phase of the Yellow Emperor 黃帝, the green wood that of the Xia dynasty 夏 (17th-15th cent. BCE), white metal that of the Shang dynasty 商 (17th-11th cent. BCE), red fire that of the Zhou dynasty 周 (11th cent.-221 BCE), black water that of the Qin 秦 (221-206 BCE), while the Han dynasty turned back the circle and adopted the element fire and the corresponding colour red. This theory of the influence of the Five Processes on history came up relatively late in the Warring States period and was brought forward by Zou Yan 鄒衍 (ca. 305-240 BCE). The theory of the regular cycle was finally established by the Former Han period scholar Dong Zhongshu 董仲舒. His eminent position in the Confucian world integrated the Five Processes theory into the public perception of the state and its position in the universe.
The imperial bibliography in the official dynastic history Hanshu lists 31 books on the Five Processes, that of the Suishu 隋書 272 books. The greatest part of these books deals with prognostication with the help of starry constellations. The bibliography on soothsayers (127 Rizhe zhuan 日者傳) in the Shiji that was written by Chu Shaosun 褚少孫, also says that the wuxing experts mainly did do mantic work. He criticizes the divinatory schools for the contradicting results of their findings and gives and example of a prognostication for an ideal wedding day. The Five-Phases diviners (wuxingjia 五行家), geomancers (kanyujia 堪輿家), activity diviners (jianchujia 建除家, using months and the twelve daily human activities), "thicket hour" diviners (congchenjia 叢辰家, using the presence of spirits during the twelve double-hours of the day), calendar diviners (lijia 歷家), diviners of the "heavenly man" (tianrenjia 天人家) and Taiyi diviners (taiyijia 太一家) all found different results.
The number five played an important part in Chinese thinking. Even Confucian philosophers resorted to it when establishing the rule of the five virtues kindheartedness (ren 仁), righteousness (yi 義), etiquette (li 禮), wisdom (zhi 智) and saintness (sheng 聖), or creating a canon of Five Classics.
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The following two tables give an overview of the cosmic correlations of the Five Processes. The first table is an extract from the medical classics Huangdi neijing 黃帝內經 (chapter 4), the second is table is a summary of the correlations as described in the Lüshi chunqiu 呂氏春秋 (Ji 紀 "Almanachs" 1-12), Huainanzi 淮南子 (chapter 3 Tianwen xun 天文訓 "Astronomy") and the Shiji 史記 (Chapter 25 Lü shu 律書 "Harmony and measurements").
||3 + 5 = 8
||2 + 5 = 7
||4 + 5 = 9
||1 + 5 = 6
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||Tai Hao 太皞
||Yan Di 炎帝
||Shao Hao 少皞
||Zhuan Xu 顓頊
||Gou Mang 句芒
||Zhu Rong 祝融
||Ru Shou 蓐收
||Xuan Ming 玄冥