Henry David Thoreau, born 1817 and died 1872, was an American writer, philosopher, essay writer and thinker who believed in the abolition of slavery, trascendentalist ideals, the importance of nature and a simple lifestyle and the importance of an individual´s capacity to self-govern and self-regulate, as well as to oppose government actions, and governments themselves, that attempt against the inherent goodness and individual principles, through civil disobedience. Although many of Thoreau´s beliefs and ideals derived from trascendentalist philosophies, Thoreau did not preach an absence of government, but rather an improvement of it through the individual´s actions and opposition to unjust or corrupt actions from the government through acts of disobedience. Among the many things that were written about by Thoreau, one that was very close to him was his opposition to the Mexican-American War, basically because he did not believe it to be right that America would be attempting to invade a foreign land, and change the life of foreigners, simply to extend its land ownership, and particularly, to extend slavery. And he might have written an essay opposing this war because he believed that individuals had the power, and the duty, to balance the power of the government, through opposition. This is why the correct answer here is A: to explain the individual´s responsibilities within a corrupt society.
