  I present the sketch of a "physicist's" derivation of the AdS/CFT
correspondence for the original pair, string theory in $AdS_5\times S^5$ vs.
${\cal N}=4$ SYM, based on the pp wave limit, and deviations from it. I show
that one can reverse the logic, and derive the action of ${\cal N}=4$ SYM, as
kinetic terms plus vertices, as well as the elements of its path integral, from
string theory on the pp wave. One can also treat consistently deviations from
the "strings on the pp wave" limit, starting from the SYM side, and a priori
reconstruct the full $AdS_5\times S^5$ geometry perturbatively, as well as full
perturbative string theory on it.
