In a crucial paper (1933), Dirac explained how classical mechanics is an emergent phenomenon of quantum mechanics: destructive interference among paths with non- extremal macroscopic actions S » ħ obliterate amplitude contributions in the path integral he introduced, leaving the extremal action S class, thus the classical action path as the dominant contribution, an observation further elaborated by Feynman in his 1942 PhD dissertation. (Further see quantum decoherence.)