  Based on the modelling of quantum systems with the aid of (classical)
non-equilibrium thermodynamics, both the emergence and the collapse of the
superposition principle are understood within one and the same framework. Both
are shown to depend in crucial ways on whether or not an average orthogonality
is maintained between reversible Schroedinger dynamics and irreversible
processes of diffusion. Moreover, said orthogonality is already in full
operation when dealing with a single free Gaussian wave packet. In an
application, the quantum mechanical "decay of the wave packet" is shown to
simply result from sub-quantum diffusion with a specific diffusivity varying in
time due to a particle's changing thermal environment. The exact quantum
mechanical trajectory distributions and the velocity field of the Gaussian wave
packet, as well as Born's rule, are thus all derived solely from classical
physics.
