“Nature is inside art as its content, not outside as its model.”

The Education of Henry Adams (1907)
Context: p>His superiority was, indeed, real and incontestable; he was the classical ornament of the anti-slavery party; their pride in him was unbounded, and their admiration outspoken.The boy Henry worshipped him, and if he ever regarded any older man as a personal friend, it was Mr. Sumner. The relation of Mr. Sumner in the household was far closer than any relation of blood. None of the uncles approached such intimacy. Sumner was the boy's ideal of greatness; the highest product of nature and art. The only fault of such a model was its superiority which defied imitation. To the twelve-year-old boy, his father, Dr. Palfrey, Mr. Dana, were men, more or less like what he himself might become; but Mr. Sumner was a different order — heroic.</p

In a letter to H. P. Bremmer (Dutch art-critic and buyer of his paintings), Paris 29 January 1914; as quoted in Mondrian, - The Art of Destruction, Carel Blotkamp, Reaktion Books LTD. London 2001, p. 75
1910's

9. The Inside and the Outside
Complexity and Contradiction in Architecture (1966)

as quoted in "The man who got it right," The New York Review of Books, Volume 60, Number 13, August 15, 2013, p. 72

Source: The Courage to Create (1975), Ch. 5 : The Delphic Oracle as Therapist, p. 99
Context: The self is made up, on its growing edge, of the models, forms, metaphors, myths, and all other kinds of psychic content which give it direction in its self-creation. This is a process that goes on continuously. As Kierkegaard well said, the self is only that which it is in the process of becoming. Despite the obvious determinism in human life — especially in the physical aspect of ones self in such simple things as color of eyes, height relative length of life, and so on — there is also, clearly, this element of self-directing, self-forming. Thinking and self-creating are inseparable. When we become aware of all the fantasies in which we see ourselves in the future, pilot ourselves this way or that, this becomes obvious.

The Origins of Art (1966)
Other Quotes
Context: What I am searching for... is some formula that would combine individual initiative with universal values, and that combination would give us a truly organic form. Form, which we discover in nature by analysis, is obstinately mathematical in its manifestations—which is to say that creation in art requires thought and deliberation. But this is not to say that form can be reduced to a formula. In every work of art it must be re-created, but that too is true of every work of nature. Art differs from nature not in its organic form, but in its human origins: in the fact that it is not God or a machine that makes a work of art, but an individual with his instincts and intuitions, with his sensibility and his mind, searching relentlessly for the perfection that is neither in mind nor in nature, but in the unknown. I do not mean this in an other-worldly sense, only that the form of the flower is unknown to the seed.

"Against Interpretation" (1964), p. 8
Against Interpretation and Other Essays (1966)

Source: 1960s, Jours effeuillés: Poèmes, essaies, souvenirs (1966), p. 359
Context: I like nature but not its substitutes. Naturalist art, illusionism, is a substitute for nature. I remember that in arguing with Piet Mondrian [in Paris, 1920's], he opposed art to nature saying that art is artificial and nature is natural. I do not share this opinion. I do not think that nature is in natural opposition to art. Art's origins are natural.

Source: 1940's, Beyond the Aesthetics (1946), p. 38