# What good is infinity?

I am becoming increasingly convinced that Wildberger's views are, if a little bizarre, at least not hopelessly inconsistent.

When I was reading the comments in the video following (MF17), somebody said something that shocked me a bit, because I was unable to give a rebuttal that I found satisfactory:

The reason I consider [the axiom of infinity] restrictive is that it forces you to accept the existence of a certain set that has all sorts of bizarre properties, like the existence of one-to-one and onto mappings from some sets to proper supersets. That screws up your attempts to assign cardinalities to sets (see the continuum hypothesis), and it doesn't even buy you much: can you have a set of all sets? Why not? The real question is, what do you think infinite sets actually buy you in terms of reasoning power?

The point about the continuum hypothesis is arguably meaningless; as I understand it CH says that it is hard to assign to each cardinality a set, but nothing about the difficulty of assigning each set a cardinality.

However, the core of the question is a bit harder for me to answer. My immediate response is it that allowing infinity gives a sort of completeness. But as the person pointed out, it doesn't finish the job, it just (dramatically) kicks down the line where the new notion of "too big" is. Introducing classes pushes it even further, but the same problem arises, I think: there is still no class of all classes.

So my next idea was, well, infinity doesn't buy me reasoning power but it does provide a satisfying source of many examples. But I'm not sure if that's true either: Wildberger's alternative to ZFC is (if consistent) a type theory, or at least it uses the language of type theory. I know very little about type theory so if you want to reference it in an answer, it would be great if you could use small words :)

With type theory on your side it's not even clear that I have a much less restrictive universe of objects which I can speak about; just a not-entirely-arbitrary boundary where sets are no longer permitted and types must take over. This could be dramatically the wrong picture, since as I said I am very new at this.

And now I'm stuck. Can anyone save Cantor's paradise for me?

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Are you aware that are theorems about finite combinatorial objects whose proofs require infinite sets? (hence there can be no "comprehensive, self-contained discipline of finite combinatorial mathematics"). For example, see this answer. –  Bill Dubuque Dec 25 '13 at 1:22
Nope, I was not. As close as I got was that I knew the finite Ramsey theorem could be proven from the infinite one. But I also know there is a finite (finitistic?) proof of the Ramsey Theorem. Thank you for bringing that to my attention; would you consider leaving it as an answer? –  Eric Stucky Dec 25 '13 at 1:25
I feel like Professor Wildberger is like the man who can't understand the reason for art, asking why abstract paintings are useful, when we have perfectly good methods for painting what we see with our own eyes. Unintuitive and bizarre results? That's why I do it! –  Jason Polak Dec 25 '13 at 2:06
@Jason Pollock, is it? :-) –  Asaf Karagila Dec 25 '13 at 2:27
@AsafKaragila: Hilarious! :) –  Jason Polak Dec 25 '13 at 3:38

I could argue against Wildeberger's philosophical views directly, but I don't really see the point.

The reason is that philosophical arguments aren't a very good method for making decisions. What works much better, in almost every case, is experimentation and evidence. If we want to figure out the best foundations for mathematics, the only effective method is to experiment: we should invent some foundations, and then figure out what mathematics you can make with it.

So what does the axiom of infinity buy you? Well, all of modern mathematics.

I can't think of a single field of mathematics that doesn't use the axiom of infinity on a regular basis, and I can't imagine how virtually any of the important results in mathematics discovered over the last century could be proven or even stated without the axiom of infinity. It is enormously conceptually helpful to be able to consider infinite collections of objects.

Overall, ZFC has been quite successful as a foundation for mathematics over the last century, and it would take an enormous amount of evidence to convince the mathematical community to switch to some other foundation.

If Wildberger wants to show that mathematics would be better off without the axiom of infinity, he will need demonstrate that his alternative foundations are either cleaner or more powerful than ZFC. The first step would be to get some set theorists interested in a workable version of set theory that doesn't include the axiom of infinity, so that they can start to investigate the mathematics that results. This is not unheard of: there has been a lot of work, for example, on the new foundations as a possible alternative to ZFC, or on non-standard analysis as a possible alternative to analysis.

But it does very little good to argue abstractly about possible shortcomings of ZFC without offering a workable alternative, where "workable" means "sufficient for developing virtually all of mathematics". If you can't get Fermat's last theorem and the Poincare conjecture, then you don't have a viable alternative to ZFC.

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Is it fair for me to say that your direct answer to the question (What good is infinity?) is "We don't really have enough information to answer the question at this time."? That seems like a perfectly reasonable response, I just want to make sure I'm not misrepresenting you. It sounds like your argument is "ZFC has worked pretty *** well. Other things we haven't tried enough to know if they work well." –  Eric Stucky Dec 25 '13 at 1:40
The axiom of infinity is even used by finitists to argue that infinity is inconsistent!! :-) –  Asaf Karagila Dec 25 '13 at 2:02
@EricStucky: I think this answer is much less non-commital than that. It sounds like it's a clear "pretty much everything" rather than a "we don't know". –  Malice Vidrine Dec 25 '13 at 4:42
@MaliceVidrine: Eh, I understand what you're seeing, but I think we're talking past each other. Yes, infinity gives us a lot (in a technical sense, "almost everything"). We know that, definitely, nobody's arguing that. But what I am saying is, and what I think Jim was getting at, is that we don't know what not-infinity gets us. It could be also "almost everything". Now, I don't know what is known; perhaps we know a lot about what not-infinity gets us. But it sounds like that's what he was saying. –  Eric Stucky Dec 25 '13 at 5:47
"What does not-infinity get us" is a rather different question than what you asked. But the answerer seems to have definite opinions on whether it's plausible to get much out of finitistic mathematics. For the record, and perhaps this is helpful to you, but what you get with not-infinity and the rest of ZF is something exactly as strong as PA. –  Malice Vidrine Dec 25 '13 at 7:50

Subjecting myself to these seven or so minutes of absolute horror, let me make a few comments on the video and on the ideas it presents.

1. The historical review is skewed and out of context. It also omits the fact that Hilbert, von Neumann, Dedekind, and many others of the great mathematicians of the last century and a half have in fact accepted and welcomed the idea of infinite sets.

The way he jumps from 1900 to 2009 is presented as if "someone brainwashed mathematicians for the last hundred years, but I have left the matrix!", and not that in fact this was a very natural progress. And that people accepted infinite sets because it is interesting.

2. These darn "axiomatics", that nobody uses. Well, sure, but I also don't use directly single oxygen atoms. Of course the fact that I breathe dioxide molecules (amongst other things), and my body uses them for me, doesn't mean that I use them.

3. Finally, judging by his video, his mind is limited to what he can see. Infinite sets don't really exist. Sure, we can't prove their physical existence, and not many mathematicians would claim otherwise. But is mathematics really about reality? Thank goodness, it's not. At least not in the last two centuries or so.

And I won't even begin to tackle deep seated philosophical issues like epistemology and so on.

Now, what do infinite sets give us? First of all, they give us rich and beautiful theories where one doesn't always have to run around and count and bound everything. Exactly by not being able to count them from top to bottom, infinite sets allow us to prove very beautiful theorems on them.

The classical, and most awesome, example is indeed Ramsey theorem. The direct proof of the finite case amounts to counting arguments which are involved and annoying; the infinite proof is neat, simple and involves the compactness theorem.

This is exactly the same argument in favor of the axiom of choice, by the way, it allows us to generalize things, and prove them neatly, using the fact that certain objects exist; otherwise one has to start running around combinatorial arguments and things get messy.

But. There is no real issue with rejecting the existence of infinite sets. That's fine. But this is a philosophical belief. And much like any other belief, one has to think about it on their own and decide. You may decide to reject evolution (despite overwhelming evidence), but that means that you are probably not going to be well received in a modern academic society. Instead, you might want to consider hanging out with the good people of "Our Lord The Creator College of Creationism" instead.

The question you ought to ask yourself is this: what is mathematics? If for you mathematics is the science of counting change correctly, doing your taxes (voodoo and witchcraft), and becoming a carpenter, then by all means. There is no need for you to accept infinite sets, at all. There is also no need for you to look at any painting except blueprints, listen to any music other than your jigsaw and hammer, or enjoy any meal other than tasteless nutrients.

If for you mathematics is an amazing abstract idea, which coincidentally ends up modeling a lot of physical phenomenon, then it's probably going to be a good idea if you accept the idea of infinite sets.

Let me finish with something that I have said more than once before. I don't have a problem with finitism, or even ultrafinitism. I have an issue with how a lot of those people argue against infinitism, allow me to refer you to Louis CK, for the point which I took from him (he's talking about evolution).

Oh yeah, in the midst of writing the answer it seems that I forgot to address the issue about the continuum hypothesis.

We can assign cardinalities to each set, that's not the problem. And infinite sets do have this certain "pathology" to them, where a proper subset can have the same cardinality as the original set.

The issue with $2^{\aleph_0}=\aleph_2$ (or any other consistent value) is that the usual axioms, id est $\sf ZFC$, cannot decide the exact value.

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Louis makes a very good point about people taking movies of their children, and posting on Facebook or the like, instead of actually watching at the time. Let me see if I can find that. Well, here is a sort of transcript facebook.com/louisckquotes/posts/452645034822971 I see, an embedded youtube thingie as well. –  Will Jagy Dec 25 '13 at 4:54
Yes. I am familiar with his work. And I agree with him, that's why I don't video my kids, or post such videos on Facebook. Or have a Facebook account. Or kids. –  Asaf Karagila Dec 25 '13 at 5:06
Alright. But what about Ziva leaving NCIS? –  Will Jagy Dec 25 '13 at 5:11
I don't watch that. So I don't know about that. –  Asaf Karagila Dec 25 '13 at 5:14
+1 for "I have an issue with how a lot of those people argue against infinitism". This is what initially made me reject Wildberger out of hand; only now with a more mature mind am I able to try to sort through what he says that does and doesn't make sense. –  Eric Stucky Dec 25 '13 at 7:21