What two things does Thoreau compare, and what is the theme in this quote, "Let your life be a counter friction to stop the machine".
Thoreau wrote "Civil Disobedience" in response to what he saw as a great injustice being supported by his government.
He compared people who mindlessly followed the law without consulting their own conscience as being akin to "machines"--no better than wood or stone. In short, people who had surrendered their humanity.
He says that if there is injustice embedded in the law, and if following that law will result in you being unjust to any human being, it is your responsibility to break the law.
In this case, the government with the injustice is the machine and the person who breaks the law is like friction. It is one person who slows the machine. Like friction, the more resistance, the more difficult it is for a machine to continue in its course. Eventually, it will stop. To do otherwise would be hypocritical, for you would be lending yourself to the wrong you condemn. In other words, you would become a cog in the service of the machine.
He was talking about slavery, and how it was the responsibility of the man with conscience to act out against it even though it was legally sanctioned by the government of the United States.