"Quantum mechanics is a more general model than classical mechanics, in the same way that Einsteinian relativity is a more general model than Galilean relativity. One picture subsumes the other. Quantum mechanics, pushed to the limit of the large, goes over smoothly into classical mechanics, whereas classical mechanics remains resolutely classical even when pushed to the limit of the small. ...Heisenberg, Schrödinger, Dirac, and the other early quantum mechanicians ...needed to peek at the classical equations in order to set the quantum equations on the right track. They needed... an idea of where the broader theory must eventually lead."