We hear this all too often, but it is completely wrong. Of course it’s fun to say, mostly for two reasons: 1) it makes the speaker seem open-minded and wise, superior to the surrounding listeners; and 2) because it does that fun word flippy thing (which I’m sure has a specific name well known to literature teachers and poets).
Simple caveat: “Absence of evidence is not PROOF of absence” is absolutely correct. I think this is what most people mean when they use the other phrase.
Second caveat: An “argument from ignorance” is different beast all together. I’m interested in situations where good observations have been made, tests have been run, etc.
“Absence of evidence is not evidence of absence” —let’s state that a different way. Failure to find evidence for thing X cannot be used as evidence that thing X doesn’t exist. Let’s apply this to a real word situation and see how it turns out.
Is there an elephant on campus? We have no sighting’s, no prints, no poo, no damage to vegetation, no noise. If “absence of evidence is not evidence of absence” then how do we comment on the probability of the presence of an elephant on campus? In fact, all we have is “absence of evidence”.
Now let’s try the unicorn. Do unicorns exist? Should I be scared of a goring when walking home late at night? Should we put aside some unicorn habitat so they can run free and mate and have baby unicorns? There is an absence of evidence for a real unicorn, so again, how do we go about gathering evidence of its absence?
All this does not mean, however, that later evidence may come to light and we must refine our ideas about the universe. That is an understood aspect of the Thing we call Science. But as it stands, if we go looking, and don’t find anything, then yes, we can use that as evidence (but not proof) that what we’re looking for isn’t out there.