  Ill-defined pinch singularities arising in a perturbative expansion in out of
equilibrium quantum field theory have a natural analogue to standard scattering
theory. We explicitly demonstrate that the occurrence of such terms is directly
related to Fermi's golden rule known from elementary scattering theory and is
thus of no mystery. We further argue that within the process of thermalization
of a plasma one has to resum such contributions to all orders as the process
itself is of non-perturbative nature. In this way the resummed propagators
obtain a finite width. Within the Markov approximation of kinetic theory the
actual phase space distribution at a given time of the evolution enters
explicitly.
