Saul Steinberg in The New York Review of Books issue dated August 15, 2002, page 32:

"The idea of reflections came to me in reading an observation by Pascal, cited in a book by W. H. Auden, who wrote an unusual kind of autobiography by collecting all the quotations he had annotated in the course of his life, which is a good way of displaying oneself, as a reflection of these quotations.  Among them this observation by Pascal, which could have been made only by a mathematician...."

Pascal's observation is that humans, animals, and plants have bilateral symmetry, but in nature at large there is only symmetry about a horizontal axis... reflections in water, nature's mirror.

This seems related to the puzzling question of why a mirror reverses left and right, but not up and down.

The Steinberg quote is from the book Reflections and Shadows, reviewed here.

Bibliographic data on Auden's commonplace book:

AUTHOR      Auden, W. H. (Wystan Hugh), 
            1907-1973.
TITLE       A Certain World; a Commonplace Book   
            [selected by] W. H. Auden.
PUBLISHER   New York, Viking Press [1970]
SUBJECT     Commonplace-books.

A couple of websites on commonplace books:

Quotation Collections and

Weblets as Commonplace Books.

A classic:

The Practical Cogitator - The Thinker's Anthology, 
by Charles P. Curtis, Jr., and Ferris Greenslet,
Houghton Mifflin Company Boston, Massachusetts
c 1962 Third Edition - Revised and Enlarged