  Based on experimental results and our previous theoretical work, a
microscopic theory of high temperature superconductivity is conjectured. In
this conjecture, superconducting and antiferromagnetic long-range orders are
driven by interlayer coupling. Strictly in two dimensions, the microscopic
Hubbard model has an (resonating valence bond) insulator-to-metal transition at
$x=x_{c}$ near optimal doping for zero temperature, leading to a quantum
critical point, and one of the crossover lines is given by the pseudogap
temperature $T^{*}$. We argue that various singular and non-Fermi liquid
properties observed near optimal doping are due to the presence of this quantum
critical point. In our conjecture, the crossover line $T^{*}$ also practically
divides the superconducting region into two, depending on the doping level with
respect to $x_{c}$. For $x \leq x_{c}$ the superconducting state has
significant antiferromagnetic correlations, while for $x > x_{c}$ it has
virtually no antiferromagnetic correlations, thus justifying the conventional
BCS theory based on the noninteracting electrons. Inelastic neutron scattering
resonance and systematically reduced superfluid density in the superconducting
state below $x_{c}$ have their natural explanations in the present scenario.
The present approach supports interlayer pair tunneling model where the
superconducting condensation energy comes from the lowering of the c-axis
kinetic energy in the superconducting state. Comparison of the present scenario
with some of the leading theories based on the Hubbard and $t-J$ models is
given. The generic features of both hole-doped and electron-doped cuprates as
well as heavy-fermion superconductors may be understood in the {\em unified}
framework within the present picture.
