It is impossible to take away peoples’ freedom just by locking them up.
Thoreau believes that just because you lock up someone's physical being doesn't mean that they are not free. He states in the passage that the townspeople outside of the prison had a more difficult wall "to climb or break through before they could be as free as [he] was." He says that he did not feel confined by the walls that imprisoned him. Throughout this passage, he is stating that he is still free even though his "flesh and blood and bones" were locked up.