The method of copper coin divination, purportedly created by Guiguzi during the Warring States period, substitutes three coins for the fifty stalks of divining grass traditionally employed in consultation. The conventional usage of copper coins dictates that the side bearing characters represents yin, while the blank side signifies yang. Tossed six times, with three instances of the blank side facing upwards indicating “old yang,” and three instances of the character-bearing side facing upwards signifying “old yin,” the combination of two blank sides and one character-bearing side represents “young yin,” whereas two character-bearing sides and one blank side denote “young yang.”
However, the rationale behind why the side with characters symbolizes yin and the blank side symbolizes yang remains elusive. Delving into antiquity yields no definitive answers. Today, without falling into superficiality, I shall briefly expound upon this matter, elucidating it for the enlightened minds.
Firstly, let us discuss ancient currency. The obverse side of a coin, referred to simply as the “face,” bears inscriptions such as weight, era, or the name of the state, serving as identifiers of the coin’s denomination. Examples include “Half-face,” “Kaiyuan Tongbao,” and “Shunzhi Tongbao.” The reverse side, or “back,” is also known as the “curtain.” Scholars have observed a variety of changes in coin reverses: those devoid of inscriptions are termed “plain backs” or “unadorned backs,” while others feature designs such as stars, moons, place names, years, or denominations, commonly seen in coins from the Qin and Han dynasties, like the “half liang” and “five zhu.” Coins from the Tang and Song dynasties onward often feature inscriptions, symbols, images, or chronograms on their reverse sides, collectively referred to as “reverse inscriptions.” If the hexagram divination truly originated from the teachings of Guiguzi during the Warring States period, then the currency of that era can be categorized into four types based on their shapes and distribution: cloth coins, knife coins, round coins, and ant nose coins (also known as ghost face coins). Cloth coins, resembling agricultural implements such as spades, circulated mainly in the capital of the Zhou dynasty and the regions at the foot of the Taihang Mountains, including Jin, Zheng, Song, and Zhao. Knife coins evolved from bronze tools of the Shang and Zhou periods and circulated primarily in eastern regions such as Qi and Yan. Round coins were circular with a hole in the center and were mainly minted in Wei and Qin. In the later Warring States period, Qin, Qi, Yan, and other states minted round coins with square holes. Ant nose coins, derived from the shape of shell coins, were made of copper, flat on one side and protruding on the other, with inscriptions in seal script resembling ghostly visages, hence their moniker “ghost face coins,” predominantly minted in the state of Chu.
Within the aforementioned historical context, several points merit attention: firstly, ancient coins typically bore inscriptions of weight, year, or state; secondly, the Qin dynasty’s “half liang” and “five zhu” coins were often smooth on the reverse side; thirdly, states such as Qin, Qi, and Yan minted round coins with square holes; fourthly, ant nose coins featured a protruding front side with seal script inscriptions.
The “Treatise on Astronomy” in the Book of Jin records: “The Zhou dynasty’s masters say: the heavens are round like a canopy, the earth is square like a chessboard,” affirming the spherical heavens and square earth, as delineated in the Book of Changes’ commentary on hexagrams. It is widely acknowledged that Chinese characters are square in shape, with oracle bone inscriptions mostly carved in yin script. Therefore, the side with characters should represent earth, squareness, and yin (especially considering that the obverse side of the ant nose money is in yin script); while the reverse side should symbolize heaven, roundness, sun, brightness, and yang.
In the “Xici Zhuan” of the Book of Changes, it states: “Heaven is lofty, earth is lowly; thus the firmament and the earth are established.” The numerical sequence of heaven and earth is as follows: heaven as one, earth as two, heaven as three, earth as four, heaven as five, earth as six, heaven as seven, earth as eight, heaven as nine, and earth as ten. Heaven pertains to yang and odd numbers, while earth pertains to yin and even numbers. In figurative terms, heaven is vast while earth is small, heaven is exterior while earth is interior. As Zhang Heng put it, “Heaven encompasses the earth’s exterior, while the earth resides within.” Taking coins as an example, during the later stages of the Warring States period, states such as Qin, Qi, and Yan had already minted round coins with square holes, with the obverse side being round and external, resembling heaven and yang; conversely, the reverse side being square and internal, resembling earth and yin.
