“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.”

— Carl Sagan

Referring to an aphorism of Martin Rees. (see Misattributed below)
Source: The Demon-Haunted World : Science as a Candle in the Dark (1995), Ch. 12 : The Fine Art of Baloney Detection, p. 221

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Carl Sagan 365

American astrophysicist, cosmologist, author and science ed… 1934–1996

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Context: When I talk of the purpose of life, I am thinking not only of human life, but of all life on Earth and of the life which must exist upon other planets throughout the universe. It is only of life on Earth, however, that one can speak with any certainty. It seems to me that all life on Earth, the sum total of life upon the Earth, has purpose. If the means were available, we could trace our ancestry — yours and mine — back to the first blob of life-like material that came into being on the planet. The same thing could be done for the spider that spun his web in the grass, and of the grass in which the web was spun, the bird sitting in the tree and the tree in which he sits, the toad waiting for the fly beneath the bush, and for the fly and bush. We are all genetic brothers. The chain of life, tracing back to that primordial day of life's beginning, is unbroken...

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“[In the Universe it may be that] Primitive life is very common and intelligent life is fairly rare. Some would say it has yet to occur on Earth.”

Stephen Hawking (1942–2018) British theoretical physicist, cosmologist, and author

“Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns — the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.
The absence of evidence is not evidence of absence, or vice versa.”

Donald Rumsfeld (1932) U.S. Secretary of Defense

Department of Defense news briefing (12 February 2002)
Variant:
Now what is the message there? The message is that there are no "knowns." There are things we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know. So when we do the best we can and we pull all this information together, and we then say well that's basically what we see as the situation, that is really only the known knowns and the known unknowns. And each year, we discover a few more of those unknown unknowns.
:It sounds like a riddle. It isn't a riddle. It is a very serious, important matter.
:There's another way to phrase that and that is that the absence of evidence is not evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something exists does not mean that you have evidence that it doesn't exist. And yet almost always, when we make our threat assessments, when we look at the world, we end up basing it on the first two pieces of that puzzle, rather than all three.
:* Extending on his earlier comments in a press conference at NATO Headquarters, Brussels, Belgium (6 June 2002) http://www.nato.int/docu/speech/2002/s020606g.htm
2000s

“I've been on what I call my UFO Tour, which means, like UFOs, I too have been appearing in small southern towns in front of a handful of hillbillies lately. I've been doubting my own existence.”

Bill Hicks (1961–1994) American comedian

Flying Saucer Tour Vol. I (2002)

“Human nature does change, and the fact that we’ve developed tools to direct that change is quintessentially human. And it must be a nearly universal concomitant to technological growth everywhere in the universe; otherwise there would be no universe. Unless we’re the only technological intelligence in the universe, Julian pointed out; so far there’s no evidence to the contrary. Maybe our own existence is evidence that we’re the first creatures to evolve far enough to hit the reset button. Somebody does have to be first.
But maybe the first is always the last.”

Joe Haldeman book Forever Peace

Source: Forever Peace (1997), p. 205

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