During the Tang Dynasty, under Emperor Wenzong's reign, the great calligrapher Liu Gongquan was known for his loyalty, integrity, and frank advice, serving as the Minister of Public Works.

At that time, an official named Guo Ning sent his two daughters into the palace. Subsequently, the emperor appointed Guo Ning as an official in Younin (present-day You County, Shaanxi). This matter caused widespread public discussion.

The emperor questioned Liu Gongquan about it, saying, "Guo Ning is the stepfather of the Grand Empress Dowager and holds the title of Grand General. He hasn't committed any major faults since he took office. Now, I've only appointed him to a small post in Younin; what's so inappropriate about that?"

Liu Gongquan replied, "Everyone is saying that Guo Ning received this official post because he offered his two daughters to the palace."

Emperor Wenzong clarified, "Guo Ning's two daughters entered the palace to accompany the Empress Dowager, not for me."

Liu Gongquan responded, "Under the suspicion of a melon patch or a plum tree, how can people distinguish the truth?" The idiom "瓜田李下" (guātián lǐxià) is derived from the ancient Yuefu poem "Junzi Xing" (The Gentleman's Conduct), which contains the lines "Do not tie your shoes in a melon patch, nor adjust your hat under a plum tree." Here, Liu Gongquan used it as a metaphor, implying that the emperor's actions could easily lead to suspicion among the people.