“I crawled back into myself all alone, just delighted to observe that I was even more miserable than before, because I had brought a new kind of distress and something that resembled true feeling into my solitude.”

“April: Come High Water”, p. 25.
A Sand County Almanac, 1949, "April: Come High Water," "April: Draba," "April: Bur Oak," & "April:Sky Dance"

"An innovator who brings order to an infinitude of equations" Quanta Magazine (2018)

Praeterita, volume I, chapter IX (1885-1889).
Context: My entire delight was in observing without being myself noticed,— if I could have been invisible, all the better. I was absolutely interested in men and their ways, as I was interested in marmots and chamois, in tomtits and trout. If only they would stay still and let me look at them, and not get into their holes and up their heights! The living inhabitation of the world — the grazing and nesting in it, — the spiritual power of the air, the rocks, the waters, to be in the midst of it, and rejoice and wonder at it, and help it if I could, — happier if it needed no help of mine, — this was the essential love of Nature in me, this the root of all that I have usefully become, and the light of all that I have rightly learned.

Quote of Friedrich, 1821; as cited in Authenticity and Fiction in the Russian Literary Journey, 1790-1840 (2000) by Andreas Schönle, p. 108, from memoirs of Vasily Zhukovsky
Variant translation: I have to stay alone in order to fully contemplate and feel nature.
This answer of Friedrich is recorded by Vasily Zhukovsky who asked the painter in 1821 to travel together to Switzerland
1794 - 1840

Source: Yoga For People Who Can't Be Bothered To Do It (1993), p. 201

Playboy interview

Variant: I came to a point where I needed solitude and just stop the machine of ‘thinking’ and ‘enjoying’ what they call ‘living’, I just wanted to lie in the grass and look at the clouds.
Source: Lonesome Traveler