  Understanding quasielastic electron- and neutrino-scattering from nuclei has
taken on new urgency with current and planned neutrino oscillation experiments,
and with electron scattering experiments measuring specific final states, such
as those involving nucleon pairs in ``back-to-back'' configurations. Accurate
many-body methods are available for calculating the response of light ($A \leq
12$) nuclei to electromagnetic and weak probes, but they are computationally
intensive and only applicable to the inclusive response. In the present work we
introduce a novel approach, based on realistic models of nuclear interactions
and currents, to evaluate the short-time (high-energy) inclusive and exclusive
response of nuclei. The approach accounts reliably for crucial two-nucleon
dynamics, including correlations and currents, and provides information on
back-to-back nucleons observed in electron and neutrino scattering experiments.
We demonstrate that in the quasielastic regime and at moderate momentum
transfers both initial- and final-state correlations, and two-nucleon currents
are important for a quantitatively successful description of the inclusive
response and final state nucleons. Finally, the approach can be extended to
include relativistic---kinematical and dynamical---effects, at least
approximately in the two-nucleon sector, and to describe the response in the
resonance-excitation region.
