《资治通鉴·汉纪·汉纪二十四》
太中大夫谷永上书说： 臣听说，依照礼仪，天子要在门外修建屏障之墙，是不想直接看见外面的情景。
太中大夫谷永上书曰： 臣闻礼，天子外屏，不欲见外也。
Imperial attendant Gu Yong submitted a memorial which stated: Your subject has heard that according to proper etiquette, the emperor would erect a walled wall outside the gate, as he did not wish to directly witness the scenes outside the palace.