  Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and
other data, we propose that superconductivity in the cuprates occurs via a
convenient matching of the spatial distribution of incommensurate spin
fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair
wavefunction; this establishes a robust causal relationship between the
lengthscale of the fluctuations and the superconducting coherence length. It is
suggested that the spin fluctuations are driven by the Fermi surface, which is
prone to nesting; they couple to the itinerant holes via the on-site Coulomb
correlation energy, which inhibits double occupancy of spins or holes. The
maximum energy of the fluctuations gives an appropriate energy scale for the
superconducting $T_{\rm c}$. Based on this model, one can specify the design of
solids that will exhibit ``high $T_{\rm c}$'' superconductivity.
