The first time I heard this quote was in field school. We'd spent the majority of the summer excavating the residence of Dr. J.H. Ward and found about nothing...though I did learn that a claw hammer will totally own century old cement...
When asked what he was going to say about the residence since we'd had such a lean collection of artifacts, Dr. Mullins told me, "absence of evidence is not evidence of absence." This was quickly followed with a rather comical debate over what a particular artifact really was to which I was told, "When you hear hoof beats, think Horses not Zebras."
I took both quotes to heart, often repeating them to myself when faced with questions in the field. I still like both, even though recently I learned that the Carl Sagan quote is actually a misquote. The full quote is listed above and can be found on Wikiquote...or better yet in his book Demon-Haunted World
To put it in context, Sagan uses this phrase in his "Baloney Detection Kit". He uses it as a tool to identify and reject an "appeal to ignorance". The phrase appears in Chapter 12, "The Fine Art of Baloney Detection" in the "The Demon-Haunted World"
"appeal to ignorance - the claim that whatever has not been proved false must be true, and vice versa (e.g., There is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist - and there is intelligent life elsewhere in the Universe. Or: There may be seventy kazillion other worlds, but not one is known to have the moral advancement of the Earth, so we're still central to the Universe.) This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence."
So, this quote didn't mean what I thought it meant. What Sagan appears to be doing here is pointing out that absence of evidence IS evidence of absence.
What does this have to do with archaeology you ask? Oh my, so much. Especially when you are dealing with pseudoarchaeology.
I've been doing research for a video that will be on Ancient Astronauts building the Pyramids, not a topic I was familiar with, and frankly the more I "learn" the more my brain hurts. What I've been finding is that Ancient Astronauts supporters use the appeal to ignorance to support their claims, up to the point of quoting Sagan. This tells me is that not only do they not know anything about ancient Egyptian civilization / building techniques, they also have never read Sagan. This bothers me since in both cases they are speaking as if from positions of authority, and five minutes on the internet can blow their arguments out of the water.
I blame von Daniken for this. The man simply refuses to see fact, even when it's place right before him. He is also a prolific author on the topic of pseudoarchaeolgoy, claiming that professional archaeologists either don't know what they are doing, or are purposefully covering up the "truth". To him I ask, what do we have to gain by hiding the truth?
Daniken likes to point to known artifacts, hieroglyphs, and paintings, claiming that Archaeologists translated them incorrectly, that they are really ancient depictions of aliens, or even parts of a spaceship. He goes as far as to say that everything we know is wrong, and we have something to gain by not telling the lay public the truth. Daniken apparently never had the benefit of someone telling him to think Horses not Zebras.
To all this I say, Ancient Astronauts supporters: you have no evidence, you have no facts, most damningly, you have no practical applied experience. When you misquote Sagan, you show you have no grasp of basic concepts. As in all things, Occam's Razor comes into play, and since your extraordinary claims cannot be backed with extraordinary evidence, you really should let it go. Have a little faith in your own species, we really are a very clever and capable ape.