“Nature is what we see,
The Hill, the Afternoon—
Squirrel, Eclipse, the Bumble-bee,
Nay—Nature is Heaven.

Nature is what we hear,
The Bobolink, the Sea—
Thunder, the Cricket—
Nay,—Nature is Harmony.

Nature is what we know
But have no art to say,
So impotent our wisdom is
To Her simplicity.”

This has also appeared in the alternate form: "What we observe is not nature itself, but nature exposed to our method of questioning."
Physics and Philosophy (1958)
Variant: What we observe is not nature itself, but nature exposed to our method of questioning.
Source: Physics and Philosophy: The Revolution in Modern Science

As quoted in Truth Against the World : Frank Lloyd Wright speaks for an organic architecture (1987) edited by Patrick J. Meehan <!-- p. 29 -->
Context: God is the great mysterious motivator of what we call nature and it has been said often by philosophers, that nature is the will of God. And, I prefer to say that nature is the only body of God that we shall ever see. If we wish to know the truth concerning anything, we'll find it in the nature of that thing.

Unsourced

Opening lines, p. 104
Variant translations:
What is God-given is called nature; to follow nature is called Tao (the Way); to cultivate the Way is called culture.
As translated by Lin Yutang in The Importance of Living (1937), p. 143
What is God-given is called human nature.
To fulfill that nature is called the moral law (Tao).
The cultivation of the moral law is called culture.
As translated by Lin Yutang in From Pagan to Christian (1959), p. 85
The Doctrine of the Mean

Source: Growing Up Absurd (1956), p. 6.

In a letter to Rudolph Steiner, c. 1921-23; as quoted in Abstract Painting, Michel Seuphor, Dell Publishing Co., 1964, p. 85
1920's

Thoughts and Aphorisms (1913), Jnana

Paris 1923
As quoted in Futurism, ed. Didier Ottinger; Centre Pompidou / 5 Continents Editions, Milan, 2008, p. 312
Quotes, 1920's