《汉书·传·外戚传上》
所以说 看人的过失，可以知道他仁与不仁，现在我和陛下衹有长公主一位姐姐，陛下隆恩让丁外人侍奉公主，丁外人应当封赏爵号。 
故曰 观过知仁 。今臣与陛下独有长公主为姊，陛下幸使丁外人侍之，外人宜蒙爵号。 
Therefore by looking at other people's error, we can know if a person is benevolent or not. Now I and Your Majesty have only one elder sister, the Princess Chang. Your Majesty has treated Ding wairen generously and let him serve the Princess. Ding wairen should be granted a title.