A. Wolf wrote: >> As I said, you can formalize the notion of soundness in Set Theory. But >> this adds nothing, except that it shows that the notion of soundness has >> the same level of complexity that usual analytical or topological set >> theoretical notions. So you can also say that "unsound" means violation >> of our intuitive understanding of what the structure (N,+,*) consists in. >> We cannot formalize in any "absolute way" that understanding, but we can >> formalize it in richer theories used everyday by mathematicians. > > You're using soundness in a different sense than I'm familiar with. > Soundness is a property of logical systems that states "in this proof > system, provable implies true". Godel's Completeness Theorem shows there > exists a system of logic (first-order logic, specifically) that has this > soundness property. In other words, nothing for which an exact and complete > proof in first-order logic exists, is false.
I'm not sure I understand this. "True" and "false" are just arbitrary attributes of propositions in logic. I read you last sentence above as saying: Given premises, which I assume "true", then any inference from them using first-order logic will be "true". But that just means I will not be able to infer a contradiction (="false"). In other words, first-order logic is consistent. Of course if I start with contradictory premises I will be able construct a proof in first order logic that proves "X and not-X" which is "false". Brent > > Soundness is particularly important to logicians because if a system is > unsound, any proofs made with that system are essentially meaningless. > There are limits to what you can do with higher-order logical systems > because of this. > > I think what you're bickering over isn't the soundness of the system. I > think it's the selection of the label "natural number", which is a > completely arbitrary label. Any definition for "natural number" which is > finite in scope refers to a different concept than the one we mean when we > say "natural number". Any finite subset of N is less useful for > mathematical proofs (and in some cases, much harder to define--not all > subsets of N are definable in the structure {N: +, *}, after all) than the > whole shebang, which is why we immediately prefer the infinite definition. > > Anna > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-list@googlegroups.com To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---