It was said that Dou, as chancellor, was hard-working and careful. In 665, as Emperor Gaozong was planning to sacrifice to heaven and earth at Mount Tai, and he had Dou and Li Ji serve as his attendants. On the way from the eastern capital Luoyang to Mount Tai, as Emperor Gaozong was passing through Puyang, Emperor Gaozong, remembering that Puyang was formerly known as Diqiu (帝丘, literally the hill of the emperor), asked Dou the reason, and Dou could not answer. Another chancellor, Xu Jingzong, happened to be nearby, and responded that Diqiu was named as such because the legendary emperor Zhuanxu had his capital there. Xu later made the comment, A high level official must be knowledgeable. When I saw that Dou Dexuan could not answer, I felt ashamed for him. When Dou heard this, he said, A person has things that he can do and things he cannot do. My ability is to know what I do not know and not pretend that I know it. The people who heard this exchange admired Dou for his honesty—particularly Li Ji, who commented, It is a good thing that Xu Jingzong is knowledgeable. But Dou Dexuan's words were also deep truths.