Related quotes
"Between Solitude and Loneliness," The New Yorker, October 15, 2016

Quotations from Gurudev’s teachings, Chinmya Mission Chicago

Source: Factotum (1975), Ch. 17
Context: I got into bed, opened the bottle, worked the pillow into a hard knot behind my back, took a deep breath, and sat in the dark looking out of the window. It was the first time I had been alone for five days. I was a man who thrived on solitude; without it I was like another man without food or water. Each day without solitude weakened me. I took no pride in my solitude; but I was dependent on it. The darkness of the room was like sunlight to me. I took a drink of wine.

Source: In the Image of Orpheus: Rilke - A Soul History

Source: Persons and Places (1944), p. 159

The Labyrinth of Solitude (1950)
Variant: Solitude is the profoundest fact of the human condition. Man is the only being who knows he is alone.
Context: Solitude is the profoundest fact of the human condition. Man is the only being who knows he is alone, and the only one who seeks out another. His nature – if that word can be used in reference to man, who has 'invented' himself by saying 'no' to nature – consists in his longing to realize himself in another. Man is nostalgia and a search for communion. Therefore, when he is aware of himself he is aware of his lack of another, that is, of his solitude.
Source: Abba's Child: The Cry of the Heart for Intimate Belonging