IV. Historical Evidence: The Logic of Duke Mu of Qin's Invasion of Jin

To truly understand how Da Yan was applied in antiquity, we must return to the Spring and Autumn period. The Zuo Zhuan records many divination cases, among which the "Invasion of Jin by Duke Mu of Qin" (15th year of Duke Xi) is a perfect template for pre-Qin interpretation logic.

1. Historical Background

In 645 BCE, the State of Jin suffered a famine and borrowed grain from Qin. Duke Mu of Qin was generous. The following year, Qin suffered a famine and asked Jin for grain. Duke Hui of Jin not only refused but also attempted to take advantage of Qin's weakness by attacking them. Enraged, Duke Mu decided to invade Jin. Before deploying, he ordered the diviner Tu Fu to perform a divination.

2. The Resulting Hexagram

"He divined it and obtained 'Gu becoming Dui' (蛊之兑). The judgment was: 'A thousand chariots are thrice repulsed; after the three repulses, a male fox is captured.'"

This involves a key ancient recording method: "Hexagram Gu becoming Dui."

Original Hexagram: Gu (山风蛊, Mountain over Wind).
Resultant Hexagram: This is debated. In the Zuo Zhuan tradition, "Gu of Dui" often refers to a specific line change or the inner trigram changing to Dui (Lake). Most commentators believe this indicated the internal trigram (the bottom three lines) changed from Xun (Wind) to Dui (Lake).

3. Tu Fu's Brilliant Deduction

Tu Fu did not simply read the text of the Zhouyi. Instead, he used the "Taking of Images" (Qu Xiang) method:

Image I: Wind Falling Below the Mountain In Gu, the upper trigram is Gen (Mountain) and the lower is Xun (Wind/Wood). Wind falling below the mountain suggests not just "decay," but the idea of "returning to roots" and "rectification." Duke Mu used this to rectify the treacherous Duke Hui of Jin.
Image II: Thrice Repulsed This is a structural deconstruction. Xun (Wind) in the lower trigram can represent order and chariots. More subtly, the nuclear hexagram contains Zhen (Thunder/Chariots). "Thrice repulsed" predicted the battle's progression: the Jin army would be pushed back three times.
Image III: Capturing the Male Fox The word Gu carries the meaning of "delusion." Duke Hui of Jin was treacherous and muddled, like a suspicious fox. From the trigrams, Gen (Upper) is a dog or a hand; Xun (Lower) is a fox. In the hexagram Gu, the Upper Mountain (Gen/Stop) presses down on the Lower Wind (Xun/Fox). When a mountain pins a fox, the result is naturally "Capturing the male fox." This "male fox" referred directly to Duke Hui of Jin.

4. Verification

In the ninth month of that year, the armies of Qin and Jin fought at Hanyuan. The Qin army won a total victory and captured Duke Hui of Jin—the "male fox."