  We suggest a straightforward approach to the calculation of the dephasing
rate in a fermionic system, which correctly keeps track of the crucial physics
of Pauli blocking. Starting from Fermi's golden rule, the dephasing rate can be
written as an integral over the frequency transferred between system and
environment, weighted by their respective spectral densities. We show that
treating the full many-fermion system instead of a single particle
automatically enforces the Pauli principle. Furthermore, we explain the
relation to diagrammatics. Finally, we show how to treat the more involved
strong-coupling case when interactions appreciably modify the spectra. This is
relevant for the situation in disordered metals, where screening is important.
