《后汉书·列传·袁张韩周列传》
袁安害怕窦宪一意孤行，于是单独上密封奏章道 ：我听说功业有些难求，因为不可预见；有些事容易判断，因为道理明摆着不必怀疑。
安惧宪计遂行，乃独上封事曰：臣闻功有难图，不可豫见；事有易断，较然不疑。
Yuan An feared that Dou Xian would act recklessly, so he secretly submitted a sealed memorial. He wrote, “I have heard that some accomplishments are hard to achieve because they are unpredictable. Some matters are easy to judge because the principles are clear and leave no room for doubt.