“The presentation of facts in logical form contributes to a confusion between discovery and proof.”

Principles of Mathematics (1903), Ch. I: Definition of Pure Mathematics, p. 5
1900s
Context: The fact that all Mathematics is Symbolic Logic is one of the greatest discoveries of our age; and when this fact has been established, the remainder of the principles of mathematics consists in the analysis of Symbolic Logic itself.

Source: A History of Mathematics (1893), p. 30, Reported in Moritz (1914)

Pg. 33.
Against Method (1975)

Source: The Structure of Scientific Revolutions (1962), VI. Anomaly and the Emergence of Scientific Discoveries, p. 52 (2012 ed.)

Section 56
The True Believer (1951), Part Three: United Action and Self-Sacrifice
Context: The readiness for self-sacrifice is contingent on an imperviousness to the realities of life.... For self-sacrifice is an unreasonable act.... All active mass movements strive, therefore, to interpose a fact-proof screen between the faithful and the realities of the world.... by claiming that the ultimate and absolute truth is already embodied in their doctrine and that there is no truth nor certitude outside it.... To rely on the evidence of senses and of reason is heresy and treason. It is startling to realize how much unbelief is necessary to make belief possible. What we know as blind faith is sustained by innumerable unbeliefs.

Source: The Wine of Violence (1981), Chapter 13 (p. 157)

The Renaissance in India (1918)
Context: To attempt to penetrate through the indeterminate confusion of present tendencies and first efforts in order to foresee the exact forms the new creation will take, would be an effort of very doubtful utility. One might as well try to forecast a harmony from the sounds made by the tuning of the instrument. In one direction or another we may just detect certain decisive indications, but even these are only first indications and we may be quite sure that much lies behind them that will go far beyond anything that they yet suggest. This is true whether in religion and spirituality or thought and science, poetry and art or society and politics. Everywhere there is, at most, only a beginning of beginnings.

Letter to Edward Byles Cowell, quoted in The Life of Edward FitzGerald, Translator of the Rubáiyát of Omar Khayyán (1947) by Alfred McKinley Terhune, p. 146.
Context: Science unrolls a greater epic than the Iliad. The present day teems with new discoveries in Fact, which are greater, as regards the soul and prospect of men, than all the disquisitions and quiddities of the Schoolmen. A few fossil bones in clay and limestone have opened a greater vista back into time than the Indian imagination ventured upon for its Gods: and every day turns up something new. This vision of Time must not only wither the poet's hope of immortality, it is in itself more wonderful than all the conceptions of Dante and Milton.

Le mal de prendre une hypallage pour une découverte, une métaphore pour une démonstration, un vomissement de mots pour un torrent de connaissances capitales, et soi-même pour un oracle, ce mal naît avec nous.
Introduction to the Method of Leonardo da Vinci (1895)