“Art is a demonstration of which nature is the proof.”

"America the Beautiful: The Humanist in the Bathtub", p. 18
On the Contrary: Articles of Belief 1946–1961 (1961)

Symmetry (1952)

§ 243
The Reasonableness of Christianity (1695)
Context: The greatest part of mankind want leisure or capacity for demonstration, nor can carry a train of proofs, which in that way they must always depend upon for conviction, and cannot be required to assent to till they see the demonstration. Wherever they stick, the teachers are always put upon proof, and must clear the doubt by a thread of coherent deductions from the first principle, how long or how intricate soever that be. And you may as soon hope to leave all the day labourers and tradesmen, the spinsters and dairy-maids, perfect mathematicians, as to have them perfect in ethics this way: having plain commands is the sure and only course to bring them to obedience and practice: the greatest part cannot know, and therefore they must believe. And I ask, whether one coming from heaven in the power of God, in full and clear evidence and demonstration of miracles, giving plain and direct rules of morality and obedience, be not likelier to enlighten the bulk of mankind, and set them right in their duties, and bring them to do them, than by reasoning with them from general notions and principles of human reason?

The Art of Persuasion
Context: This art, which I call the art of persuading, and which, properly speaking, is simply the process of perfect methodical proofs, consists of three essential parts: of defining the terms of which we should avail ourselves by clear definitions, of proposing principles of evident axioms to prove the thing in question; and of always mentally substituting in the demonstrations the definition in the place of the thing defined.

XI, 10
Meditations (c. 121–180 AD), Book XI

Source: Reflections and Maxims (1746), p. 184.

A 627, B 655 (Physico-Theological Proof Impossible)
Critique of Pure Reason (1781; 1787)

Source: Guide for the Perplexed (c. 1190), Part III, Ch.21
Context: He fully knows His unchangeable essence, and has thus a knowledge of all that results from any of His acts. If we were to try to understand in what manner this is done, it would be the same as if we tried to be the same as God, and to make our knowledge identical with His knowledge. Those who seek the truth, and admit what is true, must believe that nothing is hidden from God; that everything is revealed to His knowledge, which is identical with His essence; that this kind of knowledge cannot be comprehended by us; for if we knew its method, we would possess that intellect by which such knowledge could be acquired.... Note this well, for I think that this is an excellent idea, and leads to correct views; no error will be found in it; no dialectical argument; it does not lead to any absurd conclusion, nor to ascribing any defect to God. These sublime and profound themes admit of no proof whatever... In all questions that cannot be demonstrated, we must adopt the method which we have adopted in this question about God's Omniscience. Note it.

Source: Art, 1912, Preface, p. 7-8
Context: Art is contemplation. It is the pleasure of the mind which searches into nature and which there divines the spirit by which Nature herself is animated. It is the joy of the intellect which sees clearly into the Universe and which recreates it, with conscientious vision. Art is the most sublime mission of man, since it is the expression of thought seeking to understand the world and to make it understood.