  Following an early observation of Ignatowsky, we present a derivation of the
transformation rules between inertial systems making no other assumptions than
the existence of the latter, and show that generically these rules are
characterized by a constant of nature with the dimensions of an inverse
velocity squared having the same value in all inertial frames. No independent
postulate of the existence of an absolute velocity for light waves or other
carriers of physical information is necessary. Aside from the usual Lorentz-
and galilean transformations, our analysis also allows for the existence of
four-dimensional Euclidian transformation rules between inertial systems.
