Paul Dirac, An Extensible Model of the Electron, Proc. Roy. Soc. A268, (1962) 57-67 (jstor:2414316)

Paul Dirac, The motion of an Extended Particle in the Gravitational Field, in Relativistic Theories of Gravitation, Proceedings of a Conference held in Warsaw and Jablonna, July 1962, ed. L. Infeld, P. W. N. Publishers, 1964, Warsaw, 163-171; discussion 171-175 (spire:1623740)

One should keep the need for a sound mathematical basis dominating one’s search for a new theory. Any physical or philosophical ideas that one has must be adjusted to fit the mathematics. Not the way round.

Too many physicists are inclined to start from some preconceived physical ideas and then to try to develop them and ﬁnd a mathematical scheme that incorporates them. Such a line of attack is unlikely to lead to success. One runs into difficulties and ﬁnds no reasonable way out of them. One ought then to realize that one’s whole line of approach is wrong and to seek a new starting point with a sound mathematical basis.