We always keep as the time coordinate. Like Kaluza, Einstein and Bergmann denoted the time coordinate by
and the additional spacelike coordinate by . Einstein’s and Bergmann’s claim with regard to Kaluza can be debated; in my
reading of his paper, Kaluza as well took the five dimensional space as physical (“eine fünfdimensionale Welt”). The
conflicting claim is repeated by Bergmann ([20], p. 268).