> a more theoretically neutral choice of units would be something like centimeters

Would it? A centimeter is defined in terms of the speed of light.

This isn't me being nitpicky, everything is defined in terms of something else, and to hold a justified belief (I won't call it a fact) requires the ability to place the belief in context with other justified beliefs to see:

- If any contracictions arise

- If the new belief can be used to justify other beliefs in the system.

These systems of belief form something like a directed graph (edge=justifies), and I suspect that they are kind of clumpy. Like, the majority of the beliefs only justify their immediate neighbors in the clump (which I won't call a theory). Rare are the edges in this graph that point from one clump to another, these are achieved by experiment. The experiment we have been discussing forms an edge between one clump (optics/electrodynamics) and another clump (motion). It was contact with "the facts" that allowed this bridge to be built.

I believe you would call whichever clump is more justified "the facts" and whichever one is less justified "the model being tested".

I suspect it is impossible, though it may just be devilishly hard, to know the difference between a fact, a very well justified belief, and a strongly held oppinion. None of the research I have seen indicates that humans are good at that making this kind of judgement. So we should always be a little suspicious that something that we perceive as a fact is actually one of the other two. If you encounter an apparent contradiction out there in the wild, one approach would be to tighten your grip on your justified beliefs, treat them as facts, and then call the newcomer a lie. But there are alternatives, which I will discuss in a moment.

When I claimed that people could manipulate absolutists by injecting "facts" into the absolutist's "universe", perhaps I should have said that what they are injecting is beliefs that are likely to be misapprehended as facts, and they get injected into the system of justified beliefs that is held by the victim. For more on these techniques, see: rhetoric. (Which isn't evil by the way, as some like to paint it. Whether or not the audience accepts the edits permanently, all effective communication involves the deliberate manipulation of this justification lattice in your audiences' mind.)

Relativists (or at least myself, since I have insight into only one mind), have more options when they run into something that is dresseed up like a fact, but causes contradictions. The interloper could be:

- A lie

- A justifiable belief, but one that is justifiable in a system that is inferior to mine

- A justifiable belief, but one that is justifiable in a system that is superior to mine

I'm constantly reworking what counts as true according to my belief system, so if a lie slips in it's not as big of a deal. Unless it's a very crafty lie, I'm likely to have edited it out in a few weeks anyway (since it neither justifies nore is justified by other beliefs in the system). Most of my cycles go into evaluating whether these things that appear to me as facts form a system that is better or worse than some alternative, and to which my system is itself an alternative (hence: alternative facts).

Here "better or worse" could mean a lot of things. It could have to do with contact with reality (i.e. I want a maximally coherent system) It could have to do with beauty It could have to do with something less lofty, like whether it helps me secure food for myself tonight

Both myself and the absolutist must take something on faith. The absolutist takes it for granted that his facts are the facts. I take for granted my ability to chose a "best" belief system (which I call a set of theories, but I'm open to a new term) There's a good article called "The Raft and The Pyramid" (https://www.andrew.cmu.edu/user/kk3n/epistclass/Sosa%20-%20R...) which shows that these two approaches are equivalent in their ability to bring us closer to the truth. But they are not equivalent in how they affect our behavior.

The relativist may consider structural details of proposed facts, while the absolutist is inclined to reject them outright. This has consequences all over the place, many of which are the problems the author (of the original article) indicates. Absolutists don't consider a candidate fact in the context of its enclosing theory--er, clump of justified beliefs--so they don't get a picture of the potenial richness of the world they are rejecting, lies are just lies and that's it. They are not accustomed to evaluating the merit of whole alternative systems, so if they are delusional they're likely to just keep calling things lies and dig there heels in further. It's not lies that we should be worried about, they're going to happen, it's persistent, contageous delusion of the sort whose entry is a one-way trip. As I'll argue later, familiarizing people with the equipment of relativism (i.e. my interpretation of Feyerabend--I'll confess to not having read the whole book) is the antidote.

To answer your question:

> How can you say that this other person who manufactured a fact and inserted it into your universe was wrong?

The interloper fact (unless it's a very bad lie) will have some set of beliefs that justify it. To use my original language for a moment, every statement has a theory where it is true (that's what theories are, systems for evaluating the truth or falsity of statements and assigning labels to phenomena)--but there is no guarantee that those theories are useful. So you set your worldview down--which is scary the first time you do it--and you start importing those beliefs, and you also import the beliefs that justify them, and you keep pulling on that thread untill you see just how big of a world you're dismissing by calling the interloper fact a lie. If that world is not good as the one you set down, you go back and decide that it's not for you.

So I guess the answer is that you can't, because although it's wrong in your universe, it's right in some other. But you can still reject it, and having gone in there and checked it out first, you may now understand more about this would-be liar. You may be able to show them a better way, or failing that you may be able to work out a compromise (in behavior) that doesn't require epistemic agreement. You may also come away with a bit of respect for the opposing theory, and rather than dismissing it outright, you might reconsider it from time to time.

This only works, of course, if you know you can make it back out. Otherwise there might be a system of thought so insidious that once you import it for examination, you then can't leave. So when I go exploring other belief systems (like yours, and like the author's) the only "fact" that I take with me is this: All facts are potentially mutable, except this one. It's that safety belt that gives me this reflex for opposing your position about facts.