The onus of proof is on the person making an existential claim
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Towards more picturesque speech™
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Carl Sagan: “Absence of evidence is not evidence of absence”.
JC: “Okay, sure, but absence of evidence is definitely not evidence of presence.”
A principal of sound argument that posits that one need not prove a negative. Because it’s impossible to prove a negative. It’s hard enough to prove a positive (the best you can do is point to an absence of contradictory evidence).
The principle is evidenced as well as anywhere by Mike and Vyvyan:
In The Young Ones,[1] just before The Damned kicked off a boisterous rendition of their punk classic Nasty, Mike and Vyvyan agonised over their failure to get their new video recorder working. It is a parable for today’s uncertain times.
- Mike: Maybe you shouldn’t have poured all of that washing-up liquid into it.
- Vyvyan: It says here, “ensure machine is clean and free from dust”.
- Mike: Yeah, but it don’t say “ensure machine is full of washing-up liquid”.
- Vyvyan: Well, it doesn’t say “ensure machine isn’t full of washing-up liquid”.
- Mike: Well, it wouldn’t would it? I mean, it doesn’t say “ensure you don’t chop up your video machine with an axe, put all the bits in a plastic bag and bung them down the lavatory.”
- Vyvyan: Doesn’t it? Well maybe that’s where we’re going wrong.
It flows into legal draftery, although you would not know it from the timidity with which many modern commercial lawyers festoon their contracts. You don’t need to say what you don’t have to do in a contract. Not even for the avoidance of doubt.
Philsophically—
It would seem we have a clash of Titans here. In the red corner, we have Carl Sagan, Martin Reese and that self-inflated hot-air balloon Nassim Taleb telling us:
“Absence of evidence is not evidence of absence”.
This is a fair perspective when we haven’t yet looked very hard, or the evidence, by its nature, would be hard to see even if it were there. So there was no evidence of giant squids or, er, black swans but this wasn’t proof they didn’t exist: they just hung out in places that were very hard to observe (the deep ocean, in the case of the giant squid, pre-James Cook Australia in the case of the black swan).
But in the blue corner is no lesser mortal than Bertrand Russell and his teapot — and beside them Karl Popper, who entreat us not to believe in things for which there is no evidence, at least where you would expect there to be evidence if it were true.
Of course, much of the industry of modern cosmology has been to create hard-to-falsify (but theoretically falsifiable) theories, where evidence is intrinsically hard to collect, exactly on the premise that one can’t think of anything more obvious, because there is not any evidence for it.
JC’s observation that “the absence of evidence is, in any case, not evidence of presence” is to point out the logical asymmetry: while lack of evidence doesn’t necessarily disprove existence, it doesn’t support existence either.