> Indeed, so they redefined "nature" to mean what they wanted it to mean, rather than what it observably is.

They never "redefined" the term "nature". They actually originated it. The definition you are used to is actually a descendant, a modification, of theirs.

And the problem with claiming "what it observably is" is that (as Popper, Kuhn, Feyerabend, inter alia, argue) observation is theory-laden: a lot of what you think nature "observably" is, is how you interpret your experiences through the lens of your own theoretical committments; hence, people who have different theoretical commitments, don't observe the same things you do. The raw sense data may be the same, but the conversion of that raw sense data into words and concepts proceeds differently.

> Again, it is always very telling that all those "universals" and "innate purpose" of things somehow conspired to lead to the conclusions that the person in question wanted to achieve.

Again, why can't we levy the same argument against the positions you prefer? If realism about universalism conspires to produce conclusions that the person in question wanted to achieve, why doesn't nominalist irrealism about them likewise conspire to produce conclusions that nominalists want to achieve?

> And yes, I do see it as sophistry; specifically, as dressing up trivial and/or circular arguments with intent to confuse others about their plain meaning. Much of classical philosophy is exactly that.

I disagree. I think it is a case of being so wedded to a certain interpretive framework that one loses the ability to take any of its competitors seriously, to even realise that there are other options to consider.

> Because it's the only sensible null hypothesis.

Why should irrealism about universals be the null hypothesis? And, if irrealism about universals is the null hypothesis, why not equally so irrealism about other things, for example physical matter? Why wouldn't the same logic lead to subjective idealism as the null hypothesis?

> We can postulate existence of all kinds of things at random, why universals specifically?

Because they are part of our everyday experience. We know both individual trees, and whatever it is that all individual trees have in common which makes them trees as opposed to mountains or rivers or dogs or asteroids or whatnot–the first are particulars, the second is a universal. Obviously both exist in some sense, the question is how similar those respective senses of existence are. If you view both as "existing" in fundamentally the same sense, you end up with a strongly realist position about universals, such as Plato's; if you view universals as "existing" in a much weaker sense than particulars do, you end up in the ballpark of nominalist positions such as those of William of Ockham and Thomas Hobbes. If you aim for somewhere in-between these two extremes, you end up with the "moderate realism" of Aristotle and Aquinas.

> It doesn't matter if people agree, only if the universe agrees.

But we can't know whether the universe agrees. There are many "factual" questions to which (barring some kind of afterlife) none of us now living will ever know the answer. Is there intelligent life in the Ophiuchus Supercluster? What was Julius Caesar's last meal? Somebody once knew, but the last person to ever know that died a very long time ago, and barring some unlikely archaeological discoveries (which probably don't exist to discover), we never again will know. Or, consider modestly (as opposed to outlandishly) improbable conspiracy theories – the vast majority of them are going to be false, but there are enough of them that likely a handful are actually true–but will we ever be able to know which ones?

If there are questions to which we'll never be able to know the answer, why should we assume an unknowable answer to them must actually exist?

> Facts transcend cultures. "Universals" do not.

The concept of a tree, as opposed to any particular individual tree, is an example of a universal, and I'm not aware of any human culture lacking that concept.