Steven Novella of the Neurologica blog tackles an interesting question: Is the Search for Extra-Terrestrial Intelligence (SETI) scientific?
With regard to SETI the hypothesis is this – life arose spontaneously on Earth, there is nothing special about the Earth and therefore it is possible for life to arise elsewhere in the universe. It is possible that some of that life evolved intelligence, and some of that intelligence developed technology. One method for a technological civilization to communicate across stellar distances is through radio signals. Therefore, perhaps the Earth is being bathed at this moment with intelligent radio signals from other worlds.
Every link in that logical change is perfectly reasonable. The best way to test that hypothesis is to simply look. Looking is part of science. It is a valid way to test many hypotheses. It is not necessary to be able to prove that there are no intelligent radio sources anywhere in the universe in order for this endeavor to be properly scientific.