Henry David Thoreau

Thoreau may have been one of the most important political philosophers in the 19th century, despite having more competitors for the category than any other century.

He was a Transcendentalist, believing in looking beyond pure reason. He agreed with predecessor Jean-Jacque Rousseau that civilization could be too stifling, and one needs to get back in touch with nature.

But perhaps most importantly, he was a classic liberal, who felt that an unjust law is no law at all, that if the state oversteps its bounds, it must be forced back in line, including with Civil Disobedience, a term he popularized in his book of the same name.

Without him, we may not have had Martin Luther King, Jr., and Mohandas Ghandi as we know them, given that both felt he was a pivotal source for their own ideas.