  A microscopic model for the $N\bar N\to\pi\pi$ process is presented in the
meson exchange framework, which in the pseudophysical region agrees with
available quasiempirical information. The scalar ($\sigma$) and vector ($\rho$)
piece of correlated two--pion exchange in the pion--nucleon interaction is then
derived via dispersion integrals over the unitarity cut. Inherent ambiguities
in the method and implications for the description of pion--nucleon scattering
data are discussed.
