Is "tu quoque" always a logical fallacy? Or is it usually justified?

[teo123]

Flat-Earthers often make arguments such as "If the Earth were round, we would expect the horizon to appear to fall as we climb. It doesn't." or "If ships disappearing bottom first was caused by the Earth being round, we would expect the ships to appear to lean as they recede over the horizon. They don't. They appear to sink, but not to lean.". Is it wrong to respond to such arguments with: "The Flat-Earth Theory doesn't explain those things either. If the Earth is flat, what even is the horizon?"? At the first sight, that seems like the tu quoque logical fallacy, however, I think that in that case it is justified. The whole point of scientific debates is to find out which theory has the most explanatory power. What Flat-Earthers are doing with such arguments is precisely the opposite of that: they are trying to replace a theory which explains at least something with a "theory" that explains precisely nothing. Is it a logical fallacy to warn them about that? I don't think it is.

And suppose somebody was making this argument in an attempt to discredit the mainstream interpretation of the Croatian names of places: "Mainstream onomastics claims that, in Illyrian, consonants were doubled (geminated) after a short vowel. That's how they explain the double-n in 'Pannonia' (from *pen meaning "marsh"), that the 'n' was geminated after a short 'a'. But mainstream onomastics is also claiming that the -ap- in the river names 'Colapis' (ancient name for Kupa) and 'Serapia' (ancient name for Bednja) comes from *h2ep (water). If so, the 'a' in -ap- must have been short, so why wasn't the 'p' geminated?". Wouldn't a proper response be "I don't understand what you are imagining happened. Do you have some other explanation for the river names 'Colapis' and 'Serapia' which doesn't suffer from that problem? I think that, if there is a certain river name etymology, that is that 'Serapia' means 'flowing water', quite obviously from *ser (to flow) and *h2ep (water). Or do you have some other etymology for 'Pannonia' which doesn't suffer from the problem of the geminated 'n'?". Would that be tu quoque? Or is that an entirely reasonable response?

I've written a blog-post where I explain why I think tu quoque is usually not a fallacy.

[aroneous]

Is it wrong to respond to such arguments with: "The Flat-Earth Theory doesn't explain those things either. If the Earth is flat, what even is the horizon?"? At the first sight, that seems like the tu quoque logical fallacy, however, I think that in that case it is justified. The whole point of scientific debates is to find out which theory has the most explanatory power. What Flat-Earthers are doing with such arguments is precisely the opposite of that: they are trying to replace a theory which explains at least something with a "theory" that explains precisely nothing. Is it a logical fallacy to warn them about that? I don't think it is.

Are you sure that would be considered a tu quoque fallacy? Tu quoque is a form of ad hominem and thus can never be a valid argument. This seems more like a purely logical rebuttal, here you're simply reminding them of the burden of proof that they have unjustly tried to transfer onto you. I imagine that tu quoque would be more like, if you used some faulty mathematics to prove that the earth is round, and they do the same in attempting to prove that it is flat, but when you call them out for that they refuse to take you seriously because of your own previous mistakes (as if that would make their proof any more correct).