Einstein argued that: “If one were to put Newtonian gravitational mechanics into the form of absolutely covariant
(four-dimensional) equations, one would undoubtedly be convinced that [the] principle …rules them out, granted not
theoretically, but practically” (Einstein, 1918, p. 242; transl. from Rynasiewicz, 1999, p. 458). Within a few years, Cartan and
Friedrichs proved Einstein wrong by providing simple, four-dimensional formulations of Newtonian theory that incorporate the
equivalence principle (see, e.g., Ehlers, 1973).