Beware arguments from possibility

I'm in two minds about this.

On the one hand: yes, there's a philosophical failure mode where someone says "X is possible" meaning only that X seems plausible, and then draws far-reaching conclusions that would only follow from a much stronger notion of possibility. (Arguments from the alleged possibility of "philosophical zombies" are typically like this, for instance.)

On the other hand: prima facie, saying "X is impossible" is also an awfully strong claim: it means there's no, er, possible way to reconcile X with the other things we know. And I'm not sure both P and ~P can rightly be regarded as very strong claims in the relevant sense. I think Bob's feeling that X is plausible is evidence that X is possible, albeit sometimes very weak evidence, and in many cases there's an implicit (and not terribly bad) argument along the following lines: "For X to be actually impossible, there would need to be an actual logical contradiction between X and the other stuff we know, and there obviously isn't because there's too much vagueness in both X and the other stuff we know for a watertight argument to be available."

"possible" in this usage is always a fact about Bob (or whoever), and supports whatever conclusions in Bob's mind to the degree that Bob is correct.

Keep in mind that "evidence" is what you know on your map, not what's true in the territory. "the evidence" doesn't exist in an objective shared pool of observations, each agent has a set of evidence they use to form beliefs. "possible" in this sense, like "probability", is a statement about one's knowledge, not about the universe.

There are other uses of the word which mean closer to "a configuration of the universe that's reachable from the current state", but we probably can never access that level of truth.

I remember hearing an anecdote about a philosopher making this kind of argument for intuitionistic logic: you can think about dropping the law of excluded middle, therefore the law must not hold universally; after all, logic is about our thoughts themselves, and whatever logic governs thoughts about the possiblity of discarding the law of excluded middle must not include the law, if it is taking the hypothesis seriously. Therefore, the law of excluded middle is no law at all.

If this way of arguing worked, you could discard any law of logic you could name.

Moved to frontpage.

did you just say it's impossible to find a contradiction starting with <...>?

Isn't this often trivially true though? Such claims are called unfalsifiable. To the contrary of your point, Bob is wrong because it is impossible to find a contradiction, not because it might in fact be possible.