why does thoreau state it is impossable to bring about reform lawfully

Basically Thoreau felt that the laws were so constraining that there was no room to argue or debate them. It is Thoreau’s extremely personal response to being imprisoned for breaking the law. Because he detested slavery and because tax revenues contributed to the support of it, Thoreau decided to become a tax rebel.

Check out this link,

http://thoreau.eserver.org/wendy.html