“All ordinary expression may be explained causally, but creative expression which is the absolute contrary of ordinary expression, will be forever hidden from human knowledge.”

The Transcendent Unity of Religions (1953; revised edition 1984)

Also quoted in "Conversations with Samuel Beckett and Bram van Velde by Charles Juliet" by Nicholas Lezard, in The Guardian (23 January 2010) http://www.theguardian.com/books/2010/jan/23/conversations-samuel-beckett-van-velde
Three Dialogues (1949)

On Truth and Lie in an Extra-Moral Sense (1873)
Context: Between two absolutely different spheres, as between subject and object, there is no causality, no correctness, and no expression; there is, at most, an aesthetic relation: I mean, a suggestive transference, a stammering translation into a completely foreign tongue — for which I there is required, in any case, a freely inventive intermediate sphere and mediating force. "Appearance" is a word that contains many temptations, which is why I avoid it as much as possible. For it is not true that the essence of things "appears" in the empirical world. A painter without hands who wished to express in song the picture before his mind would, by means of this substitution of spheres, still reveal more about the essence of things than does the empirical world. Even the relationship of a nerve stimulus to the generated image is not a necessary one. But when the same image has been generated millions of times and has been handed down for many generations and finally appears on the same occasion every time for all mankind, then it acquires at last the same meaning for men it would have if it were the sole necessary image and if the relationship of the original nerve stimulus to the generated image were a strictly causal one. In the same manner, an eternally repeated dream would certainly be felt and judged to be reality. But the hardening and congealing of a metaphor guarantees absolutely nothing concerning its necessity and exclusive justification.

From Diplomacy and Art http://diplomatartist.com/diplomacy-art/, a contributer article for Diplomat Artist, October 10, 2015

in La formation scientifique, Une communication du Prix Nobel d’économie, Maurice Allais http://www.canalacademie.com/Maurice-Allais-la-formation.html, address to the Académie des Sciences Morales et Politiques (1997).
Context: Any author who uses mathematics should always express in ordinary language the meaning of the assumptions he admits, as well as the significance of the results obtained. The more abstract his theory, the more imperative this obligation.
In fact, mathematics are and can only be a tool to explore reality. In this exploration, mathematics do not constitute an end in itself, they are and can only be a means.

As quoted in "Global citizen", interview in Scouts (July/August 2010), p. 41
Context: My programme, The Art of Creative Expression, empowers young people with tools to express themselves. We teach photography, art and drama, but it's not just the medium that's important, it's about what you are trying to say.

First Dialogue; translated by Judith R. Bush, Christopher Kelly, Roger D. Masters
Dialogues: Rousseau Judge of Jean-Jacques (published 1782)

Address before the Indian Council of World Affairs, New Delhi, India, April 5, 1956, as quoted in Walter P Reuther: Selected Papers (1961), by Henry M. Christman, p. 135
1950s, Address before the Indian Council on World Affairs (1956)