In Feynman's path integral formulation, the quantum propagator K(x_f,t_f; x_i,t_i) = ∫Dx(t) exp(iS[x]/ℏ) is a sum over ALL paths connecting the endpoints, each weighted by a phase factor e^(iS/ℏ) where S = ∫(T−V)dt is the classical action. Paths near the classical trajectory (δS=0) interfere constructively; wildly deviating paths cancel. As ℏ → 0, only the classical path survives — recovering Newton's laws. For larger ℏ, more paths contribute, spreading the wavefunction. The path integral is also the foundation of quantum field theory: replace x(t) with φ(x,t) to get the field theory generating functional Z = ∫Dφ exp(iS[φ]/ℏ).