I haven't looked up the original paper myself, but I found the quote
cited as P.A.M. Dirac, Proc.R.Soc.London Ser.A 123, 714 (1929). The
qote is now known as "Dirac's dictum". Appareantly, it is from a
Dirac gave in Cambridge in 1929.
The paragraph before the "dictum" is equally interesting, because
Dirac thought that the newly developed theory of relativistic quantum
mechanics would be of no importance for chemical applications.
Georg Schreckenbach Tel: (Canada)-403-220 8204
Department of Chemistry FAX: (Canada)-403-289 9488
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