The essence of Dirac's problem was simply that: and, in general: (for non-zero and ), as everyone learns (or should learn) at school.
At which point mere mortals give up. Geniuses try harder.
(Norman McCubbin, "Beauty in physics: the legacy of Paul Dirac" , in Contemporary Physics, Volume 45, Number 4, July-August 2004, pp. 319–333)