《史记·七十列传·仲尼弟子列传》
听我授业时，颜回象个蠢笨的人，下课后考察他私下的言谈，也能够刻意发挥，颜回实在不笨。 
 回也如愚；退而省其私，亦足以发，回也不愚。 
When he listened to my teachings, Yan Hui seemed stupid, but when I inspected his private conversations after class, he was able to elaborate intentionally. Yan Hui was not stupid at all.