In the example of the Schrödinger/KdV equation, we treat the theory as equivalence of two concepts of Liouvillian integrability: quadrature integrability of linear differential equations with a parameter (spectral problem) and Liouville's integrability of finitedimensional Hamiltonian systems (stationary KdV equations). Three key objects in this field-new explicit J-function, trace formula and the Jacobi problem-provide a complete solution. The Q-function language is derivable from these objects and used for ultimate representation of a solution to the inversion problem. Relations with nonintegrable equations are also discussed.