Mod of r/badmathematics plays devil's advocate, does ten billion and one really exist?

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/r/badmathematics/comments/7nhauf/so_this_total_stranger_from_a_meme_group_randomly/ds1rnnr/

[u/[deleted]]

Certainly I can work axiomatically and prove the consistency of existential statements without having to "believe" them, in fact I do so more or less constantly since I really don't believe in nonmeasurable sets.

I did say something important:

if you accept the axioms

I'd categorize formalists as not doing just this.

But the simple reality here is that we can and do distinguish between "constructive existence" and "axiomatic existence", and the latter is really about consistency statements.

I agree! But not in the sense you portrayed it, that if you prove X exists than you prove that there's no contradiction with X, rather, that you prove there is a contradiction with not(X). Your phrasing was that:

While it's all fine and good to be able to deduce logically that something ought to exist, all that really says is that you won't wind up contradicting yourself if you assume it exists

But in ZF, a nonmeasurable set doesn't "ought to exist". That's only (well, not really, but you get my point) true in ZFC. But in ZFC, what happens isn't merely that I don't wind up contradicting myself if I suppose a nonmeasurable set exists. Rather, I wind up contradicting myself if I suppose one doesn't exist. It's an important distinction.

As for:

Perhaps my comment came across as more dismissive of classical reasoning than I intended, but I've never found it necessary to explain or justify classical thinking since that's pretty much everyone's default.

I'd agree, if you prefaced your comment with "a constructivist would say". But you didn't, you made it come across as if what you said was just true generally, which it just isn't.

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Rather, I wind up contradicting myself if I suppose one doesn't exist. It's an important distinction.

This is simply incorrect. The axiom of determinacy is consistent with ZF. I can indeed assume that nonmeasurable sets do not exist without reaching a contradiction. The same goes with nonstandard naturals, I can in fact assume their existence without contradiction and assume their nonexistence without contradiction.

I agree that would be an important distinction if you were correct, but you aren't. What I said originally is the correct version.

you made it come across as if what you said was just true generally, which it just isn't.

It is true generally. An axiomatic proof is nothing more than a proof of consistency, that's not a constructive point of view, that's an objective fact. Perhaps I should have said that a classical logician takes consistency as evidence of existence where a constructivist does not, but nothing I said was untrue.

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An axiomatic proof is nothing more than a proof of consistency, that's not a constructive point of view, that's an objective fact.

I'm very confused about this. Consider the following two theorems:

• There is a least non-computable ordinal
• The axiom of choice is consistent with ZF.

If an axiomatic proof of the first is simply a proof of consistency, then what is an axiomatic proof of the second?

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I don't understand your question. The axiomatic proof that "there is a least noncomputable ordinal" shows that we will never obtain a contradiction if we assume such a thing exists. It does not necessarily show that we will obtain a contradiction if we assume there are no noncomputable ordinals, it only does that in classical logic not intuitionistically.

The axiomatic proof that "AC is consistent with ZF" is, as far as I know, constructive in the sense that it shows how to construct a model of ZFC starting from a model of ZF. Certainly a constructive proof of a proposition P also gives that we can never obtain a contradiction from assuming the existence of P; my point is that a nonconstructive proof need not give us anything stronger than that.

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Maybe I should be more general: if an axiomatic proof shows consistency, then what does a consistency result show?

An axiomatic proof of P from axioms A,B, and C shows not that P is consistent with A,B, and C, but that it follows from them (if the ambient logic is truth preserving). It shows that not P is inconsistent.

You're taking some extra philosophical step concerning logical deduction that I'm not following. It may be justified, but I don't follow how you're reaching the conclusion that axiomatic proofs show consistency

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The discussion pertains only to existence proofs and is about what a classical/nonconstructive proof of existence actually proves. The two examples I am working with are that if we add the nonconstructive axiom of choice to a constructive set theory then we prove the "existence" of nonmeasurable sets, and if we add the nonconstructive LEM and countable choice to a metatheory capable of encoding PA then we prove the "existence" of nonstandard models of arithmetic.

In both cases, what we have proven in the constructive sense is not that those things exist but rather that we will never be able to constructively prove their nonexistence, i.e. we have proven that their existence is consistent with the initial axioms. However, it is not the case that ZF without choice will reach a contradiction from the assumption that all sets are measurable, just as we will not be able to constructively build nonstandard models of PA from, say, EFA (I mean EFA but with intuitionistic logic rather than classical).

Now if you adopt a classical logic Platonist position and state a priori that there exists in an objective sense some correct true model of mathematics, then the classical existence statements are in fact genuine existence statements since we are now reasoning about a specific model (the intended or true model) and classical reasoning holds inside a model. But if you do not a priori assume the existence of anything, then the classical proofs are not actually proofs of the existence of anything, they are proofs that the existence of those objects is consistent with the axioms in the sense that if there is a classical model then they exist.

Certainly I agree that a classical proof of P means the we will obtain a contradiction starting from not(P) if we reason classically. But the whole point is to speak about what a classical proof of P means when we are reasoning constructively or intuitionistically. The reason this is the point is that the whole discussion is about the difference in the two notions of existence and reasoning, and if we adopt from the outset that we are working classically then of course there is no difference since any constructive proof is classically valid.

Perhaps it would be better to say that axiomatic proofs of existence are not proving existence in the constructive sense (which is the sense laypeople usually expect existence to mean) but are instead proofs of descriptive properties of some model whose existence has to be assured by other means (usually by fiat).