  Even though the phenomenological relations between perception and stimulus
have been firmly established, a theoretical argument for Weber's and Fechner's
law in terms of relevant models or from statistical physics is largely missing.
We present such a discussion in terms of response theory for nonequilibrium
systems, where the induced displacement or current, which stands for the
perceived stimulus, crucially depends on the change in time-symmetric
reactivities. Stationary nonequilibria may indeed generate extra currents by
changing the dynamical activity. The argument finishes by understanding how the
extra dynamical activity logarithmically encodes the actual stimulus.
