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.”
As we get smarter, it often turns out that past evidence wasn’t in fact vague, and watertight arguments were available all along. So it seems wrong to make positive claims because the evidence looks vague to us now.
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.”
As we get smarter, it often turns out that past evidence wasn’t in fact vague, and watertight arguments were available all along. So it seems wrong to make positive claims because the evidence looks vague to us now.