So, why does FlatAssembler suppose that the PIE word "yos" existed and meant "spring"?
It's a hypothesis. There are dictionaries of proposed Indo-European etymologies. Wiktionary has a number of entries and that's quick to access. There are other more comprehensive published sources.
FlatAssembler, that is a reasonable question: what is your source for 'yos'?
I'm not seeing it on a quick search. But my area of expertise/interest in this is not about lexical items in PIE. I'm not too familiar with all of the proposed roots. I assume you have a source for it, and you might be able to explain this to LinguistSkeptic.
And why do you accept that?
I haven't "accepted" anything. I've just given advice about methodology.
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The alternative you seem to be proposing (by implication) is that we assume everything is wrong. If we do that, then there's very little to try to understand. Science is full of hypotheses, many of which are built on other hypotheses.
You wrote elsewhere that you don't like assuming things (e.g., hypotheses) as facts. That's fine. But there's also a big difference between that and actively rejecting everything. A reasonable approach in science is making contingent predictions (layered hypotheses). It's possible that the foundations of an argument are false, in which case the secondary argument is also false. That's how science works. But that doesn't mean we shouldn't do science. That means that, yes, if "yos" isn't a relevant root for whatever reason, the hypothesis proposed here is also incorrect. But by exploring these possibilities we may learn something.
What is your goal in this conversation? Do you want to learn more about hypotheses? Or do you want to just keep suggesting that any given hypothesis might be wrong? If so, you're correct. And your point has been made. And you should probably avoid the whole field of Historical Linguistics, because it's full of this stuff. That doesn't mean we can't learn anything, but apparently you don't like uncertainty, and that's most of what there is. On the other hand, there are different levels of uncertainty, and if you have trouble believing Latin was ever a spoken language, then basically all hope is lost for you finding less documented areas of Historical Linguistics (e.g., most of it) to be insightful. In that case, I am having trouble figuring out why you're here. I'm attempting to treat your questions as genuine questions, but if your position is simply that you're skeptical, then why should I try to convince you otherwise? Am I making any progress? If not, I don't mind if you don't agree with me, but why are we still discussing it? Should we continue?