How was the theory of the Five Elements generated?

【The Original Chinese Text】盖闻天地未判，其名混沌；乾坤未分，是名胚晖；日月星辰未生，阴阳寒暑未分也。在上则无雨露，无风云，无霜雪，无雷霆，不过杳合而冥冥。在下则无草木，无山川，无禽兽，无人民，不过昧昧而昏作。是时一气盘中结，于是太易生水(未有气曰太易），太初生火(有气未有体曰太初），太始生木(有形未有质曰太始），太素生金(有质未有体曰太素)，太极生土（形体已具乃曰太极）。所以，水数一，火数二，木数三，金数四，土数五。迨夫三元既极，混沌一判，胚晖乃分。轻清为天，重浊为地。二气相承，两仪既生，化而成天，其始也。或人形鸟嘴，或人首蛇身。无嗜欲，无姓名，无邦国，无君臣。巢处穴居，任其风雨，亲疏同途，莫知其父子。五谷未植，饮血茹毛，其名荡荡，其乐陶陶。及其圣贤一出（谓伏羲、神农、黄帝名），智愚两分，遂有君臣父子之分，礼乐衣冠之制。鸣呼！大道废而奸诈生，妖怪出。(是时天开于子，地辟于丑，人生于寅，始立天地之义。万物生焉，奸诈并起，妖怪腾出。)

【意译与解说】宋代理学家认为：世界未形成之前，是一片模糊，什么也没有，这种状况叫混沌，或叫太极。无中可以生有，其气轻清者，上升为天；其气重浊者，下降为地。阴极者阳生，阳极者阴生，这叫太极生两仪。两仪即阴阳，是天地的精华。由两仪又生出四象，四象即四时，四季，即金木水火。土则由太极直接下降而成，这样就产生了五行。所以《五行大义》说：“凡万物之始，莫不始于无而后有。是故易有太极，是生两仪，两仪生四序，四序生之所生也。有万物滋繁，然后万物生成也。”万物皆由水生，水极阴，阴极则阳生，所以冬至为阳之始，其月为子，数一。阴极则阳生，所以夏至为阴之始，其月在午，数二。日从东升，昼为阳，东方属木,所以数三。日从西落，夜为阴，西方属金，所以数四。土居中央，包含万物，故数五。《灵枢经》则解释为：“太一者，水之尊号，先天地之母，后万物之源。草木子实未就，人虫胎胚皆水，故数一。然水之聚而形质化，莫不备阴阳之气在中而后成。就是人禀父母阴阳生成之化，先生二肾，左肾属水，故数一。右肾属火，故数二。草木子实春生而秋成。春木秋金，故木曰三，金曰四。然水有所属，火有所藏，木有所发，金有所别，皆因土而成，故数五。”这就是阴阳五行的形成，是一切术数的根本。“天开于子，地辟于丑，人生于寅。”解释最清楚的是宋代大儒邵康节的《皇极经世》一书。他把人类世界的历史寿命，根据易理象数和古天文星象学的规律，归纳成简单的四个字，即“元、会、运、世。”一元包括十二会(指十二个月)，一会包括三十运(一月三十天），一运包括十二世（即一天十二个时辰），一世包括三十年，一个时辰分三十分。一元即是这个世界从生到死的整个时间，其演变过程要经过十二会。第一会10800年，是天由子会开始，慢慢形成。第二会10800年，形成地球。第三会则是人类慢慢繁衍的时代。此即“天开于子，地辟于丑，人生于寅”的理论来源。

【Paraphrasing and Explanation】Neo-Confucian scholars in the Song Dynasty believed that before the world was formed, there was a blurry state with nothingness. This state was called chaos, or Taiji (the Supreme Ultimate). Something can be born from nothing. The light and clear qi rises to form the sky, while the heavy and turbid qi descends to form the earth. When yin reaches its extreme, yang is born, and when yang reaches its extreme, yin is born. This is what is called that Taiji gives birth to the Two Modes. The Two Modes refer to yin and yang, which are the essence of heaven and earth. The Two Modes give rise to the Four Symbols. The Four Symbols refer to the four seasons, and also the five elements of metal, wood, water, and fire. Earth is directly formed by the descent of Taiji, and in this way, the Five Elements are generated. Therefore, "The Great Significance of the Five Elements" states: "All things begin with nothing and then come into being. Thus, in the Book of Changes, there is Taiji, which gives birth to the Two Modes. The Two Modes give rise to the four sequences, and from the four sequences, all things are born. As all things multiply and thrive, everything is formed." All things are born from water. Water represents extreme yin, and when yin reaches its extreme, yang is born. Therefore, the Winter Solstice is the beginning of yang, and the month corresponding to it is zi, which is numbered as one. When yin reaches its extreme, yang is born, so the Summer Solstice is the beginning of yin, and the month corresponding to it is wu, which is numbered as two. The sun rises in the east, and the daytime is yang. The east corresponds to wood, so it is numbered as three. The sun sets in the west, and the nighttime is yin. The west corresponds to metal, so it is numbered as four. Earth is in the center, containing all things, so it is numbered as five. "The Miraculous Pivot Classic" explains: "Taiyi is the honored title of water. It is the mother before heaven and earth and the source of all things. For plants and the seeds of plants, as well as the embryos of humans and insects, all originate from water, so it is numbered as one. However, when water accumulates, its form and substance transform, and it is imbued with the qi of yin and yang before it is formed. In the case of humans, they are formed by the yin and yang from their parents. First, the two kidneys are formed. The left kidney corresponds to water, so it is numbered as one. The right kidney corresponds to fire, so it is numbered as two. The seeds of plants sprout in spring and mature in autumn. Spring corresponds to wood and autumn corresponds to metal, so wood is numbered as three and metal is numbered as four. However, water has its own attribution, fire has its place of hiding, wood has its origin of growth, and metal has its characteristics. All these are formed because of earth, so earth is numbered as five." This is the formation of yin-yang and the Five Elements, which is the foundation of all numerology and divination. "Heaven opened in the zi period, earth was formed in the chou period, and humans were born in the yin period." The most explicit explanation can be found in "The Book of Supreme Principle Governing the World" by Shao Kangjie, a great Confucian scholar in the Song Dynasty. According to the principles of the Book of Changes, numerology, and the laws of ancient astronomical astrology, he summarized the historical lifespan of the human world into four simple characters, namely "yuan (cycle), hui (assembly), yun (fortune), and shi (age)." One yuan includes twelve hui (referring to twelve months). One hui includes thirty yun (thirty days in a month). One yun includes twelve shi (that is, twelve hours in a day), and one shi includes thirty years. One yuan represents the entire time from the birth to the end of the world, and its evolution process goes through twelve hui. The first hui lasts for 10,800 years, during which heaven begins to form starting from the zi hui. The second hui also lasts for 10,800 years, during which the earth is formed. The third hui is the era when humans gradually multiply. This is the theoretical source of the saying "Heaven opened in the zi period, earth was formed in the chou period, and humans were born in the yin period."