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How are you, if you are, responding to the critique about the use of infinity in mathematics from the constructivists?
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JSuarez said:First, there are no such thing as "constructivists"; there are a lot of variants of constructivism that differ widely in their assumptions.
Second, most forms of constructivism do not reject infinity altogether (to my knowledge, only the most extreme finitists, reject any form of infinity); what some constructivists object to is some uses of actual infinity (complete, infinite entities; some accept them up to enumerability, some go higher); but practically all accept infinite sets up to the enumerable ones.
Third: personally, I don't respond; I'm a Platonist.
JSuarez said:Third: personally, I don't respond; I'm a Platonist.
For example: is the statement A: "There exist an integer n in such that property P is satisfied" a logically sound statement if you are unable to create the algorithm to produce this integer?
What would be the syntactical equivalence?
Surely, the (possibly isomorphic to A) statement B: "(1 satisfies P) or (2 satisfies P) or (3 satisifies P) or... etc" is not a statement that can be stated in its entirety in finistic means, thus B must be rejected as a statement altogether (as the the argument goes..).
Suppose it was proven that there does not exist an algorithm which can produce n. Could A possibly be meaningful?
My question is still: how do you defend the use of the actual infinity (if you can't find a logical (syntactical) isomorphism in finitistic means), like the set of real numbers, in mathematics? Being a platonist doesn't skip this problem.
Platonism is so... 350 BC.
No, but it can be stated meaningfully in infinitary logic and, as I am yet to see a convincing argument that non-finitistic methods are harmful, I have no problem with it.
JSuarez said:Intuition is, at best, a psychological construct that may serve as an aid in science, but it's not something that can dictate what is scientifically valid or not. The strong form of the axiom of choice (there are several forms) has, indeed, non-intuitive consequences but they are also valid non-intuitive consequences (that is, they correctly follow from it). That's all there is to it; if intuition doesn't like them, too bad for it.
Jarle said:Any constructivist is a constructivist however, and they all have in common their rejection of the use of actual infinity. I believe potential infinity is allowed in most directions.
Yes, I agree, but you provided the argument that since the results "works" in the real world we should not deny the axiom. Since it clearly opposes our notions, why would it serve as a foundation for something which we consider as an extension of human logic?
As I understand it, in many forms of constructive mathematics, the axiom of choice is a theorem. The intuitive point being that it's easy to well-order "constructions".
The claim that there exist things like real numbers that you cannot write down is somewhat sketchy -- it's confusing internal and external. See Skolem's paradox for a related example.
This is quite wrong. I would say that almost all constructivists accept (actual, completed) infinities. Even an extreme finitist might accept the existence of ω = {0, 1, 2, ...}; this is a theorem of Zermelo set theory, minus the axiom of infinity, plus the negation of the axiom of infinity!
http://us.metamath.org/mpeuni/omon.html
JSuarez said:You are right, and I said it in an earlier post: only the most extreme finitists reject enumerable sets.
CRGreathouse said:This is quite wrong. I would say that almost all constructivists accept (actual, completed) infinities.
I think the term you're looking for isn't "constructivist" nor even "finitist", but "ultrafinitist".
Jarle said:How would you define an actual and completed infinity in that case?
How would you define an actual and completed infinity in that case?
JSuarez said:The discussion has, so far, revolved around the Axiom of Choice but, as far as I know, very few people questions the Axiom of Infinity, which is also an existencial statement, that explicitly introduces an (actual) infinite set in the hierarchy. More, without this axiom, it's impossible to have infinite sets.
The constructivist approach to infinity in mathematics is based on the idea that infinity is not a real, tangible quantity but rather a potential or limit. According to this perspective, infinity is not a number that can be reached or calculated, but rather a concept that represents an ever-expanding quantity.
The use of infinity in mathematics from the constructivists is often seen in the form of limits and potential infinities. For example, in calculus, the concept of a limit is used to describe the behavior of a function as the input approaches infinity. Additionally, potential infinities are often used in constructivist mathematics to describe ever-growing quantities that do not have a specific value.
The main difference between the constructivist approach to infinity and other perspectives is the idea that infinity is not a real quantity that can be reached or calculated. Instead, it is seen as a concept that represents a potential or limit. This differs from other perspectives, such as the classical approach, which sees infinity as a definite, tangible quantity.
The constructivist perspective on infinity often leads to a more rigorous and careful approach to mathematical proofs. This is because constructivists reject the use of potential infinities and instead focus on proofs that are based on finite objects and processes. This approach helps to avoid potential contradictions and ensure the validity of mathematical arguments.
Yes, infinity can still be used in practical applications in mathematics from the constructivists. While they may reject the use of potential infinities, constructivists still acknowledge the importance of the concept in certain mathematical concepts, such as limits and infinite series. Additionally, the constructivist approach can lead to new and innovative applications of infinity in areas such as computer science and physics.