The book is "the strange case of dr. jekyll and mr. hyde"
Jekyll complains that he has lost confidence in himself. The lesson had presumably had something to do with Hyde's affect on Jekyll.
The doctor seemed seized with a qualm of faintness: he shut his mouth tight and nodded. "I knew it," said Utterson. "He meant to murder you. You have had a fine escape." "I have had what is far more to the purpose," returned the doctor solemnly: "I have had a lesson—O God, Utterson, what a lesson I have had!" And he covered his face for a moment with his hands. Ch5
