Jean-Paul Sartre, La Nausée

I live alone, entirely alone. I never speak to anyone, never; I receive nothing, I give nothing… When you live alone you no longer know what it is to tell something: the plausible disappears at the same time as the friends. You let events flow past; suddenly you see people pop up who speak and who go away, you plunge into stories without beginning or end: you make a terrible witness. But in compensation, one misses nothing, no improbability or story, each too tall to be believed in cafes.

The physiologically conditioned needs are not…

… the only imperative part of man’s nature. There is another part just as compelling, one which is not rooted in bodily processes but in the very essence of the human mode and practice of life: the need to be related to the world outside oneself, the need to avoid aloneness. To feel completely alone and isolated leads to mental disintegration just as physical starvation leads to death. This relatedness to others is not identical with physical contact. An individual may be alone in a physical sense for many years and yet he may be related to ideas, values, or at least social patterns that give him a feeling of communion and “belonging”. On the other hand, he may live among people and yet be overcome with an utter feeling of isolation, the outcome of which, if it transcends a certain limit, is the state of insanity which schizophrenic disturbances represent. This lack of relatedness to values, symbols, patterns, we may call moral aloneness is as intolerable as the physical aloneness, or rather that physical aloneness becomes unbearable only if it implies moral aloneness. The spiritual relatedness can assume many forms; the monk in his cell who believes in God and the political prisoner kept in isolation who feels one with his fellow fighters are not alone morally. Neither is the English gentleman who wears his dinner jacket in the most exotic surroundings nor the petty bourgeois who, though being deeply isolated from his fellow men, feels one with his nation or its symbols. The kind of relatedness to the world may be noble or trivial, but even being related to the basest kind of pattern is immensely preferable to being alone. Religion and nationalism, as well as any custom and any belief however absurd and degrading, if it only connects the individual with others, are refuges from what man most dreads: isolation.

— Erich Fromm (Escape From Freedom)