a revealing passage in his technical work, Vilenkin describes the need for boundary conditions to restrict degrees of mathematical freedom on possible solutions to the Wheeler-DeWitt equation. He remarks: “In ordinary quantum mechanics, the boundary conditions for the wave function are determined by the physical setup external to the system under consideration. In quantum cosmology, there is nothing external to the universe, and a boundary condition should be added to eq. (9) [the Wheeler-DeWitt equation].”28
