  Using a suitable notion of principal G-bundle, defined relative to an
arbitrary cartesian category, it is shown that principal bundles can be
characterised as adjunctions that stably satisfy Frobenius reciprocity. The
result extends from G, an internal group, to G an internal groupoid. Since
geometric morphisms can be described as certain adjunctions that are stably
Frobenius, as an application it is proved that all geometric morphisms, from a
localic topos to a bounded topos, can be characterised as principal bundles.
