This question was a little vague, not sure which version of Walden he was referring to. However, generally speaking, this sentence did not appear in the main text of Walden Lake, but in the notes or last pages of some versions. If it was referring to the text of Walden Lake, then this sentence might have appeared in chapter 18 or 19. However, it should be noted that this sentence was not the core content of Walden Lake, but the metaphor and emotion made by some authors when they quoted Walden Lake.