“A man can be himself only so long as he is alone; and if he does not love solitude, he will not love freedom; for it is only when he is alone that he is really free.”

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: Orlando: A Biography (1928), Ch. 2
Context: While fame impedes and constricts, obscurity wraps about a man like a mist; obscurity is dark, ample, and free; obscurity lets the mind take its way unimpeded. Over the obscure man is poured the merciful suffusion of darkness. None knows where he goes or comes. He may seek the truth and speak it; he alone is free; he alone is truthful, he alone is at peace.

Letter to Dorothy Day (20 December 1961).
Context: Persons are not known by intellect alone, not by principles alone, but only by love. It is when we love the other, the enemy, that we obtain from God the key to an understanding of who he is, and who we are. It is only this realization that can open to us the real nature of our duty, and of right action. To shut out the person and to refuse to consider him as a person, as an other self, we resort to the impersonal "law" and to abstract "nature." That is to say we block off the reality of the other, we cut the intercommunication of our nature and his nature, and we consider only our own nature with its rights, its claims, it demands. And we justify the evil we do to our brother because he is no longer a brother, he is merely an adversary, an accused. To restore communication, to see our oneness of nature with him, and to respect his personal rights and his integrity, his worthiness of love, we have to see ourselves as similarly accused along with him … and needing, with him, the ineffable gift of grace and mercy to be saved. Then, instead of pushing him down, trying to climb out by using his head as a stepping-stone for ourselves, we help ourselves to rise by helping him to rise. For when we extend our hand to the enemy who is sinking in the abyss, God reaches out to both of us, for it is He first of all who extends our hand to the enemy. It is He who "saves himself" in the enemy, who makes use of us to recover the lost groat which is His image in our enemy.

Phaedrus as translated in the novel, p. 104
The Charioteer (1953)

A Short History of Decay (1949)

vol. 1, p. 131
The Nature and Destiny of Man: A Christian Interpretation (1941)

"Tennessee Williams" (1956), p. 97
Profiles (1990)

The Analects, Chapter I

The Wisdom of Insecurity (1951)
Context: There is no formula for generating the authentic warmth of love. It cannot be copied. You cannot talk yourself into it or rouse it by straining at the emotions or by dedicating yourself solemnly to the service of mankind. Everyone has love, but it can only come out when he is convinced of the impossibility and the frustration of trying to love himself. This conviction will not come through condemnations, through hating oneself, through calling self love bad names in the universe. It comes only in the awareness that one has no self to love.