“Both the physicist and the mystic want to communicate their knowledge, and when they do so with words their statements are paradoxical and full of logical contradictions.”

Plough, Sword, and Book : The Structure of Human History (1988), Ch. 5 : Codification, p. 123

1920s, Science and the Modern World (1925)

"How I Write", The Writer, September 1954
1950s

"Words Weird and Wonderful", in Castle of the Otter (1982), Reprinted in Gene Wolfe, Castle of Days (1992)
Nonfiction

Source: The Flame is Green (1971), Ch. 5 : Muerte De Boscaje

Introduction
Guide for the Perplexed (c. 1190), Part III
Context: To give a full explanation of the mystic passages of the Bible is contrary to the law and to reason; besides, my knowledge of them is based on reasoning, not on divine inspiration [and is therefore not infallible].... It is... possible that my view is wrong, and that I misunderstand passages referred to.... Those, however, for whom this treatise has been composed, will, on reflecting on it and thoroughly examining each chapter, obtain a clear insight into all that has been clear and intelligible to me. This is the utmost that can be done in treating this subject so to be useful to all without fully explaining it.

Source: The Burning Page (2016), Chapter 6 (pp. 64-65)

The Gospel in a Pluralist Society. Eerdmans, 1989 (reprinted 2002),19.

Source: The Age of Extremes (1992), p. 422.