The arguer's credibility does not affect the truth value of any statements, so it's always more useful to argue over why someone is wrong. You cannot guarantee that any segment of the population agrees with your credibility assessments. And these assessments should be established via appeals to factual evidence to begin with, anyhow, if we're to believe they should carry any weight at all. Otherwise, we're arguing over feelings instead of facts.
For people with wild, unsupported claims, it's better to simply take the tact that we have not yet seen evidence that establishes those claims and that what's on offer presently does not meet the necessary burden. In that argument, it really doesn't matter who is making the claim, only that the claim itself is not supported.
It is evidence which you should weigh in deciding how much credence to give their statements.
> so it's always more useful to argue over why someone is wrong.
It's a tradeoff, so there's no obvious bright line with which everyone will agree. But certainly there are some people for which some arguments or claims can be reasonably dismissed out of hand, relative to the preferences of the person making this decision.
You essentially point out that it's an easy shortcut. And it is, but there's no good reason to take it. One's feelings of credibility do not change the truth value of any statements and so arguments over it are not useful. You will end up arguing over whose feelings are superior and not over observable facts.
(Though an appeal to authority is a perfect setup for a motte-and-bailey, so it's important to be wary. Also, it is not, nor ever can be, proof, since that should hold irrespective of authority.)
Moreover, when it's an argument, you want others to agree with you. If you have to take shortcuts and infer the truth of something by proxy because you are either too lazy or not capable of evaluating the matter directly, you probably have no business arguing over it in the first place when you could instead apply your expertise to show how wrong it is.
Worse, your own correlations work against you, as you can now correlate that someone who cannot or will not do that work in making an argument, dragging irrelevant (if sometimes correlated) things into the argument becomes a signal for someone using non-factual means to persuade, which necessarily correlates quite strongly with making false claims.
In short, yes, it can be simpler and it can lead you to being right, if for the wrong reasons. But nobody else has good cause to trust it, it's vulnerable to manipulation, and it's simply better in all cases to be less lazy and fully explain why something is wrong using facts instead of opinions of someone's credibility.
I understand evaluating someone's credibility as a proxy when I have no means of investigating something. I do not understand it when we are talking about scientific assertions, which must be falsifiable in order to be scientific in the first place.
That's of course a bit black/white to say, most arguments don't hold if you push them to their extremes like that. I said it because I believe (your) arguments like "saying that a person's credibility decides the truth value of the statement requires a causal relationship" are similarly black/white.
So yes, in a way the disagreement hinges on a very strict interpretation of the word "fact" versus a less strict interpretation.
It's also to save time in evaluating claims like these. If you've spent any amount of time arguing about pseudoscientific topics (or considered pseudoscience by some), you must have found that yes you can in theory evaluate all[0] (dubious or not) claims in logical, scientific, falsifiable manner. But at some point, especially in pseudoscientific-ish topics, there will be such a great number of claims[1] that you'll find yourself spending most of the time refuting spurious claims rather than actually getting any further.
At that point, you'll never get to the strict definition of the word "fact" (at least, about the particular claim that is the topic of interest), because the claims just keep piling on faster than you can evaluate them. Allowing for a bit less strict definition, allows you to prune the evaluations greatly, and at least allows you to get somewhere.
Now, that struggle doesn't seem to take place in this thread on HN, but according to user "dumbneurologist" it has taken place before elsewhere. And you're still taking that person's word on a matter of (merely claimed) reputation as well.
So yes, there is something to say for taking someone's reputation into account. But it's a pragmatic reason. If you had all the time in the world to dig into everything as deep as you want, you wouldn't strictly need to. But since we do not live in this perfect world, we can and probably should shortcut to the more interesting points of debate by including reputation.
That's the best I can argue this, I think :) It's still a choice, however. You can still choose to only accept strict facts (including "I don't know" when more specific strict facts are unavailable).
[0] ok, not all of them
[1] part of the definition of pseudoscience, where you keep adjusting or special-casing gaps in your hypothesis when it doesn't fit the data, almost necessitates this
We're not discussing the study but logic, so that's really not germane.
> It's also to save time in evaluating claims like these.
You can save far more time by ignoring them.
Poor researchers are very common. Conspiracies to smear people are very rare. If an academic has a bad reputation, the odds are good that their reputation is deserved.
None of this is conclusive - mavericks do occasionally produce brilliant-but-unconventional research - but it's hugely relevant. There aren't enough hours in the day to give every paper the benefit of the doubt. We have to use Bayesian shortcuts and crude heuristics to filter things for credibility, otherwise we'd be drowning in a sea of junk. The occasional false-positive is a price worth paying.
I could propose in this comment a cure for cancer based on a perfectly sound a logical argument and my (lack of medical) credibility would have zero affect on the cogency of what I was saying.
But it does mean, you should probably not be convinced, or at least not send me all of your money to invest in my cancer research startup.
Also for some reason it's very easy to judge something as reasonable when we actually don't have any domain expertise. Not sure why, it seems built into most of us to feel natural.
The "ad hominem fallacy" is misused on the Internet probably more than it is applied correctly.
The reason is imperfect information. If I described here an actual, real, cure for all cancer, people shouldn't believe it because credibility is shit, it hasn't been clinically proven, and it might take 10 years to do so.
If I write in a solid proof for a millennium prize problem, you're probably going to be safe not worrying about my credibility at all because the truth will be known in pretty short order.
Usually the more uncertainty the more credibility has to be weighted.