  A non-perturbative time-dependent theory of scattering a spinless particle on
a one-dimensional $\delta$-potential barrier (no bound state) is presented,
based on an exact analysis of the asymptotes of non-stationary states in the
momentum representation. Its main results, to paraphrase John Taylor, are as
follows: first, ``... for [not] every vector $|\psi_{in}\rangle$ in
$\mathcal{H}$ there is some actual orbit $U(t)|\psi\rangle$ that is asymptotic
to the free orbit $U^0(t)|\psi_{in}\rangle$ as $t\to -\infty$; and likewise for
every $|\psi_{out}\rangle$ as $t\to +\infty$''; secondly, not all `free orbits'
$U^0(t)|\psi_{in}\rangle$ and $U^0(t)|\psi_{out}\rangle$ to which the `real
orbit' (non-stationary state) $U(t)|\psi\rangle$ tends asymptotically when
$t\to -\infty$ and $t\to +\infty$, describe free dynamics; thirdly, `free
orbits', which are asymptotes of states of `real orbits' and describe free
dynamics, form a reducible space in which a superselection rule by the sign of
the particle momentum at $t\to\mp\infty$ operates.
