The reader may gather from Chapters 4 to 6 that the sequence in which we developed the classical formalism of diffusion was to first introduce the equation of motion for dynamical variables. This was achieved through Langevin equations for position and momentum. Diffusive effects were incorporated through heat bath-induced thermal noise and systematic damping parameters. This approach follows the Heisenberg picture of quantum mechanics in which the operators evolve in time whereas the wave function stays constant. Contrast this with the Schrödinger picture in which the wave functions are taken to satisfy the time-dependent Schrödinger equation while the operators remain frozen in time.