  Given any Euclidean ordered field, $Q$, and any 'reasonable' group, $G$, of
(1+3)-dimensional spacetime symmetries, we show how to construct a model
$M_{G}$ of kinematics for which the set $W$ of worldview transformations
between inertial observers satisfies $W=G$. This holds in particular for all
relevant subgroups of $Gal$, $cPoi$, and $cEucl$ (the groups of Galilean,
Poincar\'e and Euclidean transformations, respectively, where $c\in Q$ is a
model-specific parameter orresponding to the speed of light in the case of
Poincar\'e transformations).
  In doing so, by an elementary geometrical proof, we demonstrate our main
contribution: spatial isotropy is enough to entail that the set $W$ of
worldview transformations satisfies either $W\subseteq Gal$, $W\subseteq cPoi$,
or $W\subseteq cEucl$ for some $c>0$. So assuming spatial isotropy is enough to
prove that there are only 3 possible cases: either the world is classical (the
worldview transformations between inertial observers are Galilean
transformations); the world is relativistic (the worldview transformations are
Poincar\'e transformations); or the world is Euclidean (which gives a
nonstandard kinematical interpretation to Euclidean geometry). This result
considerably extends previous results in this field, which assume a priori the
(strictly stronger) special principle of relativity, while also restricting the
choice of $Q$ to the field of reals.
  As part of this work, we also prove the rather surprising result that, for
any $G$ containing translations and rotations fixing the time-axis $t$, the
requirement that $G$ be a subgroup of one of the groups $Gal$, $cPoi$ or
$cEucl$ is logically equivalent to the somewhat simpler requirement that, for
all $g\in G$: $g[t]$ is a line, and if $g[t]=t$ then $g$ is a trivial
transformation (i.e. $g$ is a linear transformation that preserves Euclidean
length and fixes the time-axis setwise).
