《韩非子·难三》
弦不调和，曲不清楚，属于中期的责任，这才是中期用来侍奉昭王的。
弦不调，弄不明，中期之任也，此中期所以事昭王者也。
If the strings are out of tune, the tune is unclear, this is the middle term’s responsibility. This is how the middle term is supposed to serve King Zhao.