Extra-terrestrial Life and Defining Existence

You cannot prove the absence of something never before detected.

This assertion is part of a larger debate I've come across as to whether or not extra-terrestrial life exists.

On the surface, it seems to make sense -- we don’t know everything, so wouldn't it be a hasty generalization to say that extra-terrestrial life doesn’t exist? But then again, couldn't this argument be used as justification for the possible existence of pixies, leprechauns, sea monsters, Santa Claus, whatever extraordinary entity you can come up with? I'm at a loss as to what conclusions to draw from this.

What are your thoughts? Is this argument a form of fallacious reasoning? If so, what is it?

The statement is accurate, at least I cannot see any flaws with it. We cannot prove that X does not exist if proving the absence of X requires complete knowledge of the universe. In fact, if disproving X requires any knowledge we don't have, we can't do it. For example, if we claim there is a the Lochness Monster is in the lake, the only way to disprove is to have complete knowledge of the lake (that is, be able to search every inch of the lake). The only thing this means is that we cannot prove these things don't exist; it is in no way evidence that they do exist.

Those who believe in magic often attempt to use this line of "reasoning" to add credibility to their claim, resulting in the argument from ignorance . "You can't prove that leprechauns don't exist, so this is evidence that they do exist!"

This situation shares some similarities with Dynamo, the magician (from https://www.logicallyfallacious.com/questions/iW8ddAY8/what_is_the_fallacy_here.html ).

With Dynamo, the argument was essentially:  Dynamo didn't offer a natural explanation for his demonstrations and the reader couldn't come up with a natural explanation either, therefore, there is no natural explanation and therefore it must be all supernatural – the notion being that the reluctance of one individual to provide an explanation and inability of another individual to offer one must mean that there is no natural explanation.

The situation here with extra-terrestrial live is much the same: our lack of evidence of extra-terrestrial life must mean that it does not exist – it just means that we can't prove it DOES exist.

Each situation is an example of proving non-existence or attempting to prove a negative.  Our inability to prove the negative doesn't demonstrate the opposite.

With respect to pixies, Santa and the rest, we have a weak analogy .  The examples are similar in that they represent elements that folks accept on faith rather than because of evidence; they are dissimilar in that we can find counter evidence to demonstrate that those examples don't make sense.  For example, there's lots to argue against a chubby guy in a red suit sliding up and down multiple chimneys delivering gifts worldwide ... all on the same night.  (If nothing else, we'd need to explain why FedEx, UPS, and the rest haven't invested in a fleet of sleighs to replace their aircraft and trucks!)

Then, of course, we have a definitional problem to avoid the ambiguity fallacy and equivocation .  I suspect someone who has narrowly escaped a shark attack might suggest that sea monsters actually do exist!  Also, with vehicles like Curiosity determining that ancient Mars was most likely an entirely habitable place for microbial life way back when, the field of astrobiology could well be on its way to demonstrating that some form life did exist there, but I suspect microorganisms aren't what the initial debate was about – hence the need for clear definition of terms.

Looks to me like a simple attempt to prove a negative, which is a fallacy. You can disprove evidence of something, but you cannot prove a negative,

"You cannot prove the absence of something never before detected."

This isn't really an answer so much as a little perspective.

You mentioned pixies. Of course, there's no evidence that such things exist anywhere.

But extraterrestrial life is a little different. Subtract "extraterrestrial," and you're left with life. The amazing thing is, we have abundant evidence of life - right here on Earth.

So we know that "life" is not only possible, it's a fact. The next step is to try and figure out if life exist anywhere else but Earth. This leads us to a couple interesting options:

1. Evolution is a natural process, similar to gravity, therefore life has a tendency to evolve on planets with certain characteristics.

2. The evolution of life on Earth is a fluke; it couldn't possibly happen again, therefore there is no life on the trillions of planets in the universe other than Earth. (Personally, I think this is as far-fetched as arguing that life on other planets is common.)

3. If you believe in God, then you know God created life on Earth because the Bible says so. However, he didn't create life anywhere else, because....well what do you know? The Bible doesn't comment on that one way or another.

If you have a closed shoebox that may or may not contain a whoopee cushion you can prove its absence (or presence), though it hasn't previously been detected, by opening the box.  This seems to suggest the statement above needs refinement in order to be reasonable. 

When you have a hard to examine area (or dimension) in which the object may or may not exist then the statement becomes reasonable though, perhaps, not very useful.