  Fully-microscopic No-core Shell Model (NCSM) calculations of all stable $s$
and $p$ shell nuclei are used to determine realistic $NN$ interaction JISP16
describing not only the two-nucleon data but the binding energies and spectra
of nuclei with $A\leq 16$ as well. The JISP16 interaction, providing rapid
convergence of the NCSM calculations, is obtained in an {\em ab exitu} approach
by phase-equivalent transformations of the JISP6 $NN$ interaction.
