“Rotten wood cannot be carved; a wall of dirty earth will not receive the trowel. This Yu! – what is the use of my reproving him?”
朽木不可雕也，粪土之墙不可圬也，于予与何诛。
Source: The Analects, Gong Ye Chang V, Chapter 10
Author: Confucius
Confucius on the Unteachable Student: Rotten Wood and a Wall of Earth
