  The Klein-Gordon equation is shown to be equivalent to coupled partial
differential equations for a sub-quantum Brownian movement of a ''particle'',
which is both passively affected by, and actively affecting, a diffusion
process of its generally nonlocal environment. This indicates circularly
causal, or ''cybernetic'', relationships between ''particles'' and their
surroundings. Moreover, in the relativistic domain, the original stochastic
theory of Nelson is shown to hold as a limiting case only, i.e., for a
vanishing quantum potential.
