  The mechanism of the initial inflation of the universe is based on
gravitationally coupled scalar fields $\phi$. Various scenarios are
distinguished by the choice of an {\it effective self--interaction potential}
$U(\phi)$ which simulates a {\it temporarily} non--vanishing {\em cosmological
term}. Using the Hubble expansion parameter $H$ as a new ``time" coordinate, we
can formally derive the {\it general} Robertson--Walker metric for a {\em
spatially flat} cosmos. Our new method provides a classification of allowed
inflationary potentials and is broad enough to embody all known {\it exact}
solutions involving one scalar field as special cases. Moreover, we present new
inflationary and deflationary exact solutions and can easily predict the
influence of the form of $U(\phi)$ on density perturbations.
