in Thoreau’s Journal:

Must be out-of-doors enough to get experience of wholesome reality—as a ballast to thought and sentiment. Health requires this relaxation, this aimless life. This life in the present. Let a man have thought what he will of Nature in the house—she will still be novel outdoors. I keep out of doors for the sake of the mineral, vegetable, and animal in me….My thought is a part of the meaning of the world, and hence I use a part of the world as a symbol to express my thought.