"What can't be proven?... I often get into debates, especially religious, where the point gets to someone saying "You can't disprove it, so I'll just keep believing it." My point is this: Lack of evidence, by default, points to non-existence, it does not give rise to limitless possibilities.
I can't disprove unicorns don't fly around Pluto, but it doesn't mean they're there.
I'm usually at a loss for words when people throw that argument forward. How would you deal with that objection?"
I agree such people are using faulty logic, and I have had similar experiences. But I would differ with the questioner on one point. He says, "Lack of evidence, by default, points to non-existence, it does not give rise to limitless possibilities". I would say that absence of evidence is not evidence of absence. Indeed, the lack of disproof does give rise to limitless possibilities. We live in a reality that may ultimately be far more ancient and vast that we can ever know - perhaps infinitely so. Many things are possible - but that isn't the point. The point is, why should someone believe in one specific possibility without any positive proof therein?
The first thing I would note in answering that question is this:
Theism is not the belief that God is possible.
Theism is not the belief that God is likely.
Theism is the belief that God is real - that she/he/it does, in fact, exist.
Given that, we then have to ask ourselves how things would work if we were to have a similar belief about anything and everything that could not be disproved. Clearly, there are many claims and possibilities that, while possible on their own, are contradictory with other equally possible claims that have not been disproved. Therefore, it would be madness to accept every possibility that has not been disproved as real.
On my philosophy site, there is a conversation I held with another person which I've titled "How Do You Determine Truth?". In it, I say that we basically have three options for dealing with claims for which there is no proof or disproof:
1) Believe them all unless or until they are disproved.
2) Believe those things we like believing and disregard the rest.
3) Believe in none of them unless or until they are proven.
I've already noted that #1 would require a person to hold an infinite variety of contradictory claims as true. The most common response seems to be #2, even if only subconscious. Even we skeptical minded folks are guilty of this from time to time about various beliefs we take on in our lives. But only #3 is consistent with genuine reason. Of course, the above simplifies the matter considerably since, in reality, things are rarely either 100% proved or 100% disproved. But the simple delineation of these three options illustrates the essence of the matter.
Therefore, perhaps a more practical way to look at it would be as Carl Sagan suggested: Extraordinary claims require extraordinary evidence. I like to note the implication of this, that ordinary claims require ordinary evidence - thus, the degree of belief in a claim should be proportionate to the degree of evidence for a claim. And, of course, all positions should be provisional and open to reassessment in the light of new evidence.
But perhaps your conversant is someone of a more romantic approach. In this case, it might be more effective to point out the inherent arrogance of making claims about things for which we have no evidence. More effective that accusations of arrogance would be to speak positively of the humility required to acknowledge our limitations to know when we have not received verifiable evidence. Thus, the lack of that humility is implied with the opposite position. This approach is more fully explored in my essay titled, "The Humanist Contemplative".
Thanks for writing! :)