I've written posts like this before, and no doubt will again. But I think it's a drum worth beating.

Here's the thing about ad hominem and its obverse, argument from authority: they're not always wrong. Overall, I'd guess (without making any claims of having actual evidence) that they're right more often than not.

In a perfect world, we'd have full information about everything, all the time, and could evaluate any claim purely on its merits. But of course we don't live in that world. So we *have* to trust people who know more about a particular subject than we do, and who have a record of talking about the subject honestly ... and we're wise to *dis*trust people who are demonstrably ignorant, or who have a record of lying.

I know a whole lot about gene regulation, a fair amount about gene-disease prediction and gene-drug interaction, and a little about everything else that falls under the bioinformatics umbrella. Thanks to my former career, I remember a great deal about emergency medicine and infectious disease epidemiology, but while that's still useful knowledge, it's out of date. And I know a little bit about paleontology, purely as a hobby.

That's about it. On those subjects, particularly those at the top of the list, I'm trustworthy. People who haven't studied them at all should believe what I say.

On *everything else* ... I'm at best a well-informed layman, and often not even that. Like everyone else: nobody can possibly know more than a tiny sliver of everything there is to know. There aren't enough hours in a day, days in a year, or years in a lifetime to do any more.

You also have people—a *lot* of people—who are proudly, willfully ignorant, but talk endlessly on the subjects they know the least about. Most creationists, antivaxers, and climate change deniers fall into this category. They take their cues from the much smaller number of people who are knowledgeable in the subjects at hand, but are deliberately lying for ideological reasons. These people know enough to craft convincing lies which the rest then repeat at length.

When you're dealing with qualified experts in a field not your own, who have given no reason to think they're habitual liars, the *best source of knowledge* is what they say. If the experts disagree, the best you can do is listen to what most of them say. The majority may be wrong, and the minority may be right—but that's for them to hash out. Kibitzers are almost guaranteed not to make any meaningful contribution to the conversation.

And when you're dealing with people who *have* shown they're habitual liars, or who proudly proclaim their ignorance but nonetheless have a strong opinion, by far the wisest course is to dismiss their claims out of hand. Ignore them if you can, mock them if you like, fight them if you must. But never let them pretend their voices are equal to those of people who have a meaningful say. Both-sides-ism is a fatal trap.