"Absence of evidence is not evidence of absence" is simply false, however much fun it may be to say. Let us suppose, for instance, that we had indeed scanned ov

"Absence of evidence is not evidence of absence" is simply false, however much fun it may be to say. Let us suppose, for instance, that we had indeed scanned over the entire universe and found that there was no evidence for life. Would we then be justified in saying "the absence of evidence is not evidence of absence"? Of course not. What other evidence of absence do you expect to uncover, other than a lack of evidence for existence? Do you expect to go out into space and have some sort of space god hand you a genuine, bona-fide certificate of nonexistence of something? It is important not to overestimate the strength of the absence of evidence, but setting the strength to zero is just as wrong. If we look in many places where we expect to find life, because we have a theory that life is easy and essentially appears the cosmological instant it is possible, and we have a theory that the conditions for life are easy and abundant, yet we look out into the universe and see no evidence of this, yes, we are completely justified in adjusting our understanding of the probabilities involved. The more of the universe we see, and the more it lacks various sorts of life (all the way from the simplest, sparsest bacteria-type life up to galaxy-spanning intelligences, each to their own degree) the more we are justified in coming to conclusions about their probabilities. Another thing people have problems with in this debate is understanding the difference between justified beliefs and true beliefs. If we hypothetically examined all but one-trillionth of the observable universe and found it bereft of any other life, we would be justified in the conclusion that the most likely scenario is that there isn't any. We could end up being wrong if that remaining one-trillionth just so happened to contain other life, but we would still be justified, having collected extensive "absence of evidence". We can not claim that we have seen enough of the universe to know that it is true that there is no other life, and we have not seen enough of it to be justified in claiming there is no bacterial life, but I do feel at this point we are pretty justified in claiming that at least this galaxy and the nearest ones do not have any galaxy-spanning, multi-million-year civilizations in them, for instance. Maybe they're there and they just don't do anything visible, but that's a pretty precise claim.