  With the advent of the quantum theory of the electron around 1927/28  Weyl abandoned his gauge theory of 1918. He did so because in the new quantum theory a different kind of gauge invariance associated with  Dirac’s theory of the electron was discovered which, as had been  suggested by Fock (1926) and London (1927), more adequately accounted for the conservation of electric   charge.[92]  Why did Weyl hold on to his gauge theory for almost a decade despite  a preponderance of compelling empirical arguments that were mounted  against it by Einstein, Pauli and   others?[93]  In one of Weyl’s (1918/1998) last letters to Einstein concerning his  unified field theory, Weyl made it clear that it was mathematics  and not physics that was the driving force behind his unified  field   theory.[94] 