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\begin{quote}  $N \approx \, \frac{1}{4}\, f_i \, L $.   \end{quote}  This states that the number of communicative civilizations in our Galaxy is just the product of the chance of emergence of intelligent life times the average lifetime of a civilization's communicative phase. It seems hard to be able to put strong theoretical constraints on both these factors. However, one can turn the problem around, and in the final post of this series I will show how the current search for radio signals from extraterrestrial intelligence (\href{http://www.seti.org/}{SETI})  can be used to put some important constraints on the two factors $f_i$ and $L$, and how this information can be used to make bold predictions about the future of humankind. %\href{http://en.wikipedia.org/wiki/Fermi_paradox}{Fermi}