Because I have a finite amount of time in the world. Because we need to decide which policy to enact (or not) today and not in 20 years. Because producing and evaluating arguments takes time, and -- most importantly -- because bad actors exist.
> The idea that I would throw out someone's argument because I've decided that it must be wrong in advance seems ludicrous.
Witness e.g. Microsoft trolling standards committees into non-existence back in the day. I'm sure they had an endless supply of valid and brilliantly illustrated technical points to consider, but at some point you have to say "you're a bad actor, and I'm going to start disregarding your arguments because you're making the decision process intractable and, like it or not, something needs to be done eventually".
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Regarding infomercials:
> The reason I don't waste my time vetting the arguments in infomercials is because I'm not currently interested in a juicer, not because of a belief that people who make infomercials are liars, no matter how consistent their arguments or verifiable their premises.
Right. You have an a priori opinion that you don't need a juicer (or whatever), and you stick with that instead of wasting time considering the commercial's many valid arguments about how buying a juicer will totally enhance your life.
edit: it's not that the infomercial guys are liars per se. It's just that you've decided that evaluating their arguments for why their product will make you happier probably isn't worth your time. And in most cases, I claim that most people make that decision precisely because the source is a paid advertisement. That's an ad hom dismissal of the argument that juicers save marriages (or whatever, can you tell I don't watch infomercials? :-) ).
> Those reasons came after listening to their arguments, not before.
It's the particular person, not any of his theories per se. If one of these theories was mentioned by another person (even someone I didn't know), I would probably investigate if I found the time. But this particular person proved his signal-to-noise ratio is too high long ago, so I stopped listening. That's ad hom, but it's also a cost-effective. I don't want to spend my finite time on this planet objectively analyzing the evidence for every conspiracy theory my friend happens to latch on to. And I think that's rational, even though it's ad hom. And the reason is that evaluating the veracity of arguments is time consuming, and I have finite time.