Reductio on all sides

Scenes from a Multiverse on the 9th:

Rosenberg’s comment:

Hello! Today’s fallacy is Reductio ad absurdum, which was invented by the Gauls on a Thursday about 2000 years ago.

The entire exchange is somewhat unhinged, but it seems that Blue is arguing implicitly:

I’ve never seen a space elf.
∴ Space elves don’t exist.

which relies on the (unspoken) assumption that

(A) If space elves exist, I would have seen them.

(but Blue hasn’t seen them, therefore, by modus tollens, they don’t exist).

Green is attempting to refute that by arguing that the implications of Blue’s claim lead to absurd consequences — by generalizing Blue’s claims to

If you’ve never seen X, then X doesn’t exist.

and switching the discussion to a new X, the cells of the body. This leads to a dispute on what it means to see something, in which Green falls back on ever more absurd auxiliary hypotheses.

To analyze Blue’s claim a reductio ad absurdum fallacy, turning on a false dichotomy, we need to see that (A) is logically equivalent to

(A’) Either space elves don’t exist or I’ve seen them.

and (A’) is, indeed, a false dichotomy. But Green doesn’t attack that and instead generates fresh absurdities of his own.

Not a triumph of logic on either side.

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