  It is often suggested that extraterrestial life sufficiently advanced to be
capable of interstellar travel or communication must be rare, since otherwise
we would have seen evidence of it by now. This in turn is sometimes taken as
indirect evidence for the improbability of life evolving at all in our
universe. A couple of other possibilities seem worth considering. One is that
life capable of evidencing itself on interstellar scales has evolved in many
places but that evolutionary selection, acting on a cosmic scale, tends to
extinguish species which conspicuously advertise themselves and their habitats.
The other is that -- whatever the true situation -- intelligent species might
reasonably worry about the possible dangers of self-advertisement and hence
incline towards discretion. These possibilities are discussed here, and some
counter-arguments and complicating factors are also considered.
