For the first time in human history, scientists can realistically search for extraterrestrial life. We may find simple life nearby-- on Mars, Europa, Titan, Enceladus, and perhaps elsewhere. What most people really want to find, however, is advanced life, life we can marvel at, relate to, even communicate with.
Currently, the only way we can really search for that kind of life is through SETI, which is limited to turning up what we can call technological intelligence. A civilization might be vibrant and brilliant-- on the order of ancient Rome or China, or Europe before 1900-- but SETI will never find it. So, can we say anything about technological intelligence?
Well, we know it's possible, because we're here. We can say there's probably not another such civilization in our general neighborhood, else we'd probably know it by now. We can say no one civilizarion has yet spread throughout our galaxy, else we'd likely know it. So, we can begin to put some hard parameters on technological intelligence. The challenge now is to begin to tighten those parameters.