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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 is  hard tobe able to  put strong theoretical constraints on both these factors. However, one can turn the problem around, and in the \href{https://www.authorea.com/25815}{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. Next: \href{https://www.authorea.com/25815}{The Fermi Paradox}