  We present a method to recover the shape and amplitude of the power spectrum
of mass fluctuations, P(k), from observations of the high redshift \lya forest.
The method is motivated by the physical picture of the \lya forest that has
emerged from hydrodynamic cosmological simulations and related semi-analytic
models, which predicts a tight correlation between the \lya optical depth and
the underlying matter density. We monotonically map the QSO spectrum to a
Gaussian density field, measure its 3-d P(k), and normalize by evolving
cosmological simulations with this P(k) until they reproduce the observed power
spectrum of the QSO flux. Imposing the observed mean \lya opacity as a
constraint makes the derived P(k) normalization insensitive to the choice of
cosmological parameters, ionizing background spectrum, or reionization history.
Thus, in contrast to estimates of P(k) from galaxy clustering, there are no
uncertain "bias parameters" in the recovery of the mass power spectrum. We test
the full procedure on SPH simulations of 3 cosmological models and show that it
recovers their true mass power spectra on comoving scales ~1-10/h Mpc, the
upper scale being set by the size of the simulation boxes. The procedure works
even for noisy (S/N ~ 10), moderate resolution (~40 km/s pixels) spectra. We
present an illustrative application to Q1422+231; the recovered P(k) is
consistent with an \Omega=1, \sigma_8=0.5 CDM model. Application to large QSO
samples should yield the power spectrum of mass fluctuations on small scales at
z ~ 2-4. (Compressed)
