Why level downward to our dullest perception always, and praise this as common sense? The commonest sense is the sense of men asleep, which they express by snoring. (Conclusion.8)
Thoreau feels that too many of us live at the level of "dullest perception." We're stuck in the conventional, the normal. We may as well be asleep, if we don't question and challenge what everybody else considers to be "common sense."