  We propose a novel approach to intrinsic decoherence without adding new
assumptions to standard Quantum Mechanics. We generalize the Liouville equation
just by requiring the dynamical semigroup property of time evolution and
dropping the unitarity requirement. With no approximations and specific
statistical assumptions we find a generalized Liouville equation which depends
on two characteristic time t1 and t2 and reduces to the usual equation in the
limit t1 = t2 --> 0. However, for t1 and t2 arbitrarily small but finite, our
equation can be written as a finite difference equation which predicts state
reduction to the diagonal form in the energy representation. The rate of
decoherence becomes faster at the macroscopic limit as the energy scale of the
system increases. In our approach the evolution time appears, a posteriori, as
a statistical variable as if time evolution would take place randomly at
average intervals t2, each evolution having a time width t1. A generalized Tam
Mandelstam inequality is derived. The relation with previous work by Milburn is
discussed. The agreement with recent experiments on damped Rabi oscillations is
described.
