  This is an exploratory study of the lightest non-singlet scalar $q\bar q$
state on the lattice with two dynamical quarks. Domain Wall fermions are used
for both sea and valence quarks on a 16^3*32 lattice with an inverse lattice
spacing of 1.7 GeV. We extract the scalar meson mass 1.58(34) GeV from the
exponential time-dependence of the dynamical correlators with $m_{val}=m_{sea}$
and N_f=2. Since this statistical error-bar from dynamical correlators is
rather large, we analyze also the partially quenched lattice correlators with
$m_{val}$ not equal $m_{sea}$. They are positive for $m_{val}>=m_{sea}$ and
negative for $m_{val}<m_{sea}$. In order to understand this striking effect of
partial quenching, we derive the scalar correlator within the Partially
Quenched ChPT and find it describes lattice correlators well. The leading
unphysical contribution in Partially Quenched ChPT comes from the exchange of
the two pseudoscalar fields and is also positive for $m_{val}>=m_{sea}$ and
negative for $m_{val}<m_{sea}$ at large t. After the subtraction of this
unphysical contribution from the partially quenched lattice correlators, the
correlators are positive and exponentially falling. The resulting scalar meson
mass 1.51(19) GeV from the partially quenched correlators is consistent with
the dynamical result and has appreciably smaller error-bar.
