From Standards of the Guqin by Juni L. Yeung, pages 45-48 (tentative).
Traditional Aesthetics for Guqin Music
Guqin has been tied with the Yin-Yang School of thought from the earliest records of history as the representative of silk (plucked string) instruments, as well the symbol of the five sounds on each of its strings. By determining which string (or tone) dictates the melody, one can determine the subject of the message the musician is trying to convey. For example, anecdotes of Master Wen of Zheng (Shi Wen 師文) and Master Kuang (Shi Kuang師曠) of Jin both cautioned their kings not to listen to music of the Shang pitch (and later the Jue and Zhi pitch), and caused season-changing chaos as they were forced to play anyway. This relation between tone quality and the rest of nature is a backbone of Chinese musical philosophy, and as much as later composers denied such fantastical tales and refused to believe it, the system is still often cited in full at the preface or as part of music theory in manuscripts. More