“To be a philosopher is not merely to have subtle thoughts, nor even to found a school, but so to love wisdom as to live according to its dictates, a life of simplicity, independence, magnanimity, and trust. It is to solve some of the problems of life, not only theoretically, but practically.”
This quote in “Walden” refers to how most philosophers spend there time thinking about the world, that they have deep thoughts. This has become the well known definition of philosophers. But Thoreau disagrees, he says that philosophers have to love wisdom, live simple and that to be a true Philosopher you have to solve problems in life be going out and doing them, not just to think about them. That is what he considers a real philosopher to be, someone who changes the world. He devoted his life to this and since we are still reading his book today it means that he most definitely changed his world.