《列子·黄帝》
老子在半路上仰天长叹道： 起初我以为你是可以教导的，现在看来不可教导了。 
老子中道仰天而叹曰： 始以汝为可教，今不可教也。 
Lao Zi stood in the middle of the road looking up at the sky and sighed deeply, saying: At first, I thought that you could be taught, but now it seems that you cannot be taught.