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Just because magnus is a world class expert at chess does not necessarily mean he is good at detecting a cheater. Furthermore I would argue that taking an experts “gut feeling” as evidence is a terrible argument.
The phrase you used has a well-known meaning. If that wasn't what you meant, then you shouldn't have used it.
Magnus may not be great at spotting a cheater, but his expertise in this game suggests that he could be, and adding that fact to an existing body of evidence isn't even close to committing an appeal to authority fallacy. To bring up that fallacy here is just lazy thinking.
OP:
> He is widely regarded as the greatest chess player to have ever lived. His opinion on what it takes to play high level chess is worth taking extremely seriously.
- A: Magnus is a great chess player
- B: Magnus claims cheating because "gut"
- C: Therefore Hans cheated
The argument rests solely on the fact that since he's an expert his word should be taken "seriously," heavily implying that Hans cheated.
> To bring up that fallacy here is just lazy thinking.
I disagree and you're not going to convince me otherwise with statements like this one.
As for his "gut" feeling being part of the argument, you're not quoting anyone here; you are again intentionally misrepresenting what others are saying.
If you're going to analyze an argument someone else is making, be honest about it or there is really no point in discussing anything with you.