Henry David Thoreau (1817-1862) was an American author, poet, and polymath who, along with Ralph Waldo Emerson, was one of the leading transcendentalists of his time. Thoreau's most famous works are his simple living memoir Walden and his essay Resistance to Civil Government, in which he lays out the framework for civil disobedience. Thoreau, whose works would influence Gandhi and MLK, died young at the age of 44 after long battles against tuberculosis and bronchitis. 

"I am grateful for what I am and have. My thanksgiving is perpetual. It is surprising how contented one can be with nothing definite - only a sense of existence."

from Letters to Various Persons, Ticknor and Fields, 1865, p. 145

(h/t Wikiquote)