according to this excerpt from walden, in which subject area did the transcendentalist thinker thoreau most likely find his inspiration?

this small lake was of most value as a neighbor in the intervals of a gentle rain-storm in august, when, both air and water being perfectly still, but the sky overcast, mid-afternoon had all the serenity of evening, and the wood thrush sang around, and was heard from shore to shore. a lake like this is never smoother than at such a time; and the clear portion of the air above it being, shallow and darkened by clouds, the water, full of light and reflections, becomes a lower heaven itself so much the more important. from a hill-top near by, where the wood had been recently cut off, there was a pleasing vista southward across the pond, through a wide indentation in the hills which form the shore there, where their opposite sides sloping toward each other suggested a stream flowing out in that direction through a wooded valley, but stream there was none. that way i looked between and over the near green hills to some distant and higher ones in the horizon, tinged with blue.

human behavior
nature
civil war
history
nonconformity