Used to be mainly the resort of religious types trying to squash reasonable doubt. Today, most frequently used by Darwinists trying to convince themselves that all those “missing links” reall exist. Rob Sheldon writes to say,
But absence of evidence IS evidence of absence. We do it all the time in astronomy. We go looking for something, say, supernovae and find only one every other year. We argue that we would have seen it if it were there, so the fact we didn’t see it means it really wasn’t there. From that we compute a rate of supernovae production in the galaxy, and so forth.
And sometimes the thing we are looking for is never found. Cosmic strings. Magnetic monopoles. From that we don’t say “they must not exist” but we say instead, “The upper limit for the density of magnetic monopoles is one per gazillion cubic kilometers.”
So negative as well as positive identifications are significant. If we are looking for missing links and don’t find them, then we need to say, “There cannot be more than x missing links per million years. Evolution predicts y. The absence of evidence is evidence of a strong upper limit on the number of intermediate forms, and the speed with which evolution must change species.”