  Historians recently rehabilitated Einstein's "physical strategy" for General
Relativity (GR). Independently, particle physicists similarly re-derived
Einstein's equations for a massless spin 2 field. But why not a light
\emph{massive} spin 2, like Neumann and Seeliger did to Newton? Massive
gravities are bimetric, supporting conventionalism over geometric empiricism.
Nonuniqueness lets field equations explain geometry but not \emph{vice versa}.
Massive gravity would have blocked Schlick's critique of Kant's synthetic
\emph{a priori}. Finally in 1970 massive spin 2 gravity seemed unstable or
empirically falsified. GR was vindicated, but later and on better grounds.
However, recently dark energy and theoretical progress have made massive spin 2
gravity potentially viable again.
