Henri-David Thoreau is among my favorite writers and philosophers. He was not afraid of living an authentic life and stayed true to himself until the end of his own existence.
Thoreau was a precursor. You might have studied Walden, his most famous book. It still fascinates me. Walden is a call to our animal side. Being one with our nature is respectfully living off nature, the essence of environmentalism. To do so, the author advocates simple living, which we rebranded nowadays as "minimalism".
Emancipator, he coined the term Civil Disobedience, which would later inspire Gandhi to lead India toward independence, or Luther King Jr to fight against racial segregation. Civil Disobedience is the root of nonviolence. Here is my favorite quote from the book:
I am too high born to be propertied,
To be a second at control,
Or useful serving-man and instrument
To any sovereign state throughout the world.
Thoreau - Civil Disobedience and Other Essays
HDT was not only a man of words but also a man of action who constantly reinvented himself: abolitionist, naturalist, tax resister, development critic, surveyor, historian... But what really stands out from Thoreau's life, to me, is his thirst for self-sustainability. Thoreau was among the few last free men.