>Yes, this is philosophy of science and technically an axiom. But it's not an arbitrary one. It's saying that "it exists <=> something would be different about the world if it didn't exist <=> it's in principle observable (though its impact) <=> can be studied by science". Call it my lack of imagination, but I don't believe it's even possible to disagree with this axiom; alternatives don't make any kind of sense.

There are several logical holes in the axiom.

The first big one is that it implies logic is applicable to the universe, i.e. that the universe must abide by logical necessities (and even more so, by logical necessities the way we perceive them). Setting aside the fact that we cannot prove logic's applicability logically (it's itself an axiom), QM has given us many non-classical-logic compliant observations.

Then, there's the assumption that: "it exists <=> something would be different about the world if it didn't exist". That's again an assumption. I could just as well consider that (the fact of its existence turning from false to true aside) the world could be the same whether something exists or not.

This presupposes when it should instead prove: that existence necessitates difference in the world.

Third, "it's in principle observable (though its impact) <=> can be studied by science" -- whether something can be studied by science or not shouldn't be a criterion for its existence.

Black holes existed for billions of years before they could be "studied by science", or before science even had any understanding of them. Only in retrospect we say that they "can be studied by science". Did the not exist when we couldn't say that, and exist now?

Or they did exist and we only lately came to find out about them, and that we can actually study them? If it's the latter, why wouldn't that -- the possibility of existence- be the case for tons of others things (e.g. sub-plank length stuff)? Because our current theories say so? Or because we thing our current theories are the be-all end-all we'll come up with?

Fourth, one could argue that something could still exist, have impact, and
not be observable. E.g. if it's impossible to make it not exist, or look at that level (so that we have no way to measure its impact). E.g. some entities or energies at the sub-Plank scale level could hold the universe together and be essential substrate. But we just have no way to know about it, because we can't look at that level (and our theories can't even go at that level).

>At this point you could correctly observe that since the universe would look precisely no different whether or not there are smiley faces on quarks - there's not a bit of information distinguishing these two hypotheses - my claim is absolutely meaningless and a waste of time.

The claim would be a waste of time to try to verify.

It could still very well be ontologically true -- which is what we're discussing.

That something that can't be verified in any way doesn't exist is a logical jump. What we can say for certain is that it could or could not exist, but we can't verify it.

We can't even say that it has no impact to us, because without being able to verify it, how can we be sure it has no impact? At best we can say "it has no impact as far as we can tell".

But it could still have a huge impact -- the same way the Quarks you've mentioned have a huge impact, had a huge impact even before we knew existed and before we could measure them, and would have have a huge impact even if we remained forever at the 15th century level of physical knowledge.