Final answer:
Thoreau's first thought upon being imprisoned in "Civil Disobedience" is that he considers the prison a foolish institution.
Step-by-step explanation:
In "Civil Disobedience," Thoreau's first thought upon being imprisoned is that he considers the prison a foolish institution. He expresses his disagreement with the idea of government imprisoning individuals for their beliefs or acts of conscience. Thoreau questions the authority and legitimacy of the prison system and criticizes its confinement of those who exercise their freedom of action and thought in opposition to unjust laws.
Learn more about Thoreau's thoughts on imprisonment