Why is it irrelevant when you assume a world where the agent who has to make the decision knows more than they actually know? Decision theory is about making decisions based on certain information that known.
I think you’ve lost the chain a bit here. We’re just discussing to what extent probability theory does or does not extend various forms of logic. The actual conditions in the real world do not affect that. Now obviously if it only extends it in conditions that do not hold in the real world, then that is important to know; but if that were the case then “probability theory extends logic” would be a way too general statement anyhow and I hope nobody would be claiming that!
(And actually if you read the argument with Chapman that I linked, I agree that “probability theory extends logic” is a misleading claim, and that it indeed mostly does not extend logic. The question isn’t whether it extends logic, the question is whether propositional and predicate logic behave differently here.)
But again all of this is irrelevant because nobody is claiming anything like that! I mentioned a finite universe, where predicate logic essentially becomes propositional logic, to illustrate a particular point—that probability theory does not extend propositional logic in the sense Chapman claims it does. I didn’t bring it up to say “Oho well in a finite universe it does extend predicate logic, therefore it’s correct to say that probability theory extends predicate logic”; I did the opposite of that! At no point did I make any actual-rather-than-illustrative assumption to the effect that that the real world is or is like a finite universe. So objecting that it isn’t has no relevance.
I haven’t studied the surrounding math but as far as I understand according to Cox’s Theorem probability theory does extend propositional calculus without having to make additional assumptions about finite universe or certain things being known.
Cox’s theorem actually requires a “big world” assumption, which IINM is incompatible with a finite universe!
I think this is getting off-track a little. To review: Chapman claimed that, in a certain sense, probability theory extends propositional but not predicate logic. I claimed that, in that particular sense, it actually extends both of them equally well. (Which is not to say that it truly does extend both of them, to be clear—if you read the argument with Chapman that I linked, I actually agree that “probability theory extends logic” is a misleading claim, and that it mostly doesn’t.)
So now the question here is, what are you arguing for? If you’re arguing for Chapman’s original claim, the relevance of your statement of Cox’s theorem is unclear, as it’s not clear that this relates to the particular sense he was talking about.
If you’re arguing for a broader version of Chapman’s claim—broadening the scope to allow any sense rather than the particular one he claimed—then you need to exhibit a sense in which probability theory extends propositional logic but not predicate logic. I can buy the claim that Cox’s theorem provides a certain sense in which probability theory extends propositional logic. And, though you haven’t argued for it, I can even buy the claim that this is a sense in which it does not extend predicate logic [edit: at least, in an uncountable universe]. But, well, the problem is that regardless if it’s true, this broader claim—or this particular version of it, anyway—just doesn’t seem to have much to do with his original one.
I think you’ve lost the chain a bit here. We’re just discussing to what extent probability theory does or does not extend various forms of logic. The actual conditions in the real world do not affect that. Now obviously if it only extends it in conditions that do not hold in the real world, then that is important to know; but if that were the case then “probability theory extends logic” would be a way too general statement anyhow and I hope nobody would be claiming that!
(And actually if you read the argument with Chapman that I linked, I agree that “probability theory extends logic” is a misleading claim, and that it indeed mostly does not extend logic. The question isn’t whether it extends logic, the question is whether propositional and predicate logic behave differently here.)
But again all of this is irrelevant because nobody is claiming anything like that! I mentioned a finite universe, where predicate logic essentially becomes propositional logic, to illustrate a particular point—that probability theory does not extend propositional logic in the sense Chapman claims it does. I didn’t bring it up to say “Oho well in a finite universe it does extend predicate logic, therefore it’s correct to say that probability theory extends predicate logic”; I did the opposite of that! At no point did I make any actual-rather-than-illustrative assumption to the effect that that the real world is or is like a finite universe. So objecting that it isn’t has no relevance.
Cox’s theorem actually requires a “big world” assumption, which IINM is incompatible with a finite universe!
I think this is getting off-track a little. To review: Chapman claimed that, in a certain sense, probability theory extends propositional but not predicate logic. I claimed that, in that particular sense, it actually extends both of them equally well. (Which is not to say that it truly does extend both of them, to be clear—if you read the argument with Chapman that I linked, I actually agree that “probability theory extends logic” is a misleading claim, and that it mostly doesn’t.)
So now the question here is, what are you arguing for? If you’re arguing for Chapman’s original claim, the relevance of your statement of Cox’s theorem is unclear, as it’s not clear that this relates to the particular sense he was talking about.
If you’re arguing for a broader version of Chapman’s claim—broadening the scope to allow any sense rather than the particular one he claimed—then you need to exhibit a sense in which probability theory extends propositional logic but not predicate logic. I can buy the claim that Cox’s theorem provides a certain sense in which probability theory extends propositional logic. And, though you haven’t argued for it, I can even buy the claim that this is a sense in which it does not extend predicate logic [edit: at least, in an uncountable universe]. But, well, the problem is that regardless if it’s true, this broader claim—or this particular version of it, anyway—just doesn’t seem to have much to do with his original one.