---Rudyard Kipling
"The Conundrum of the Workshops,"
published 1890, full text on the web at
the Kipling Society,
Bartleby.com,
the Poetry Lover's Page,
Wocky Jivvy,
etc., and in numerous
anthologies.
This is not actually an epigraph in Operator Commutation Relations by Palle E. T. Jorgensen and Robert T. Moore, D. Reidel, Dordrecht / Boston / Lancaster, 1984,
but it is quoted in the Preface there as an epigraph in Functional Analysis and Semi-groups by Einar Hille, New York, American Mathematical Society, Providence, 1948.