# Why do we need to learn Set Theory?

I was planning to write some article for the Mathematics magazine of our college and it occurred to me that it will be a good idea to write about the impact and importance of Set Theory.

I plan writing an article that precisely explains why Set Theory is important in Mathematics. I plan to divide the article into two main sections. The first section would explain the Importance of Set Theory in Mathematics and the second section would explain the Impact of Set Theory in Mathematics. Though I realize that the second section logically belongs to the first but I plan to give a separate section just to emphasize the point.

The problem that I am facing now is lack of material. To accomplish my goal I need a great deal of material to read and so I want the help of my fellow Stack Exchange users. To be precise, my question is the following,

What is the importance and impact of Set Theory in Mathematics?

Or,

Why do we need to learn Set Theory?

Note:-

If this question seems to be too broad to answer then please notify me about the problem in the comment. I will try my best to reword it to meet the community standard.

Edit:- Along with all the answers that is provided below see this paper.

• I have a hard time imagining how your classmates intend to talk about groups, rings, and fields without using ideas from set theory. Dec 20, 2014 at 7:08
• You want to write an article, do some research. Don't come here and ask us for the answer, so you could write your article. Dec 20, 2014 at 9:41
• Why are you writing an article about something you don't know about? Why don't you choose a topic you're more comfortable with? Dec 20, 2014 at 9:45
• Then you should study set theory for several years and then find out the answer for yourself! Dec 20, 2014 at 9:47
• It doesn't seem to be a research article that user170039 wants to write, so I don't see what's wrong with asking us about our opinions as to what the main reasons are to study set theory. Dec 20, 2014 at 10:14

Why study set theory?

We like to think that mathematics developed from the need of our ancients to count things. I have four sheep, you have sixteen camels, my tribe has ten dozens of men, you have six hundred wives... etc. etc. But if you look closely, counting how many things you have of a certain type, first required you have be able and collect them into one collection. The "collection of all sheep I have", or the "collection of men in my tribe", and so on.

Sets came to solve a similar problem. Sets are collections of mathematical objects which themselves are mathematical objects.

This, of course, doesn't mean that we should learn set theory just for that purpose alone. The applications of set theory are not immediate for finite collections, or rather sufficiently small collections. We don't need to think about pairs or sets with five elements as particular objects. Whatever we want to do with them we can pretty much do by hand.

Sets come into play when you want to talk about infinite sets. Infinite sets collect infinitely many objects into one collection. The set of natural numbers, the set of finite sets of sets of sets of natural numbers, the set of sets of sets of sets of sets of sets of irrational numbers, etc. Once you establish that mathematical objects can be collected into other mathematical objects you can start analyzing their structure.

But here comes the problem. Infinite sets defy our intuition, which comes from finite sets. The many paradoxes of infinity which include Galileo's paradox, Hilbert's Grand Hotel, and so on, are all paradoxes that come to portray the nature of infinity as counterintuitive to our physical intuition.

Studying set theory, even naively, is the technical spine of how to handle infinite sets. Since modern mathematics is concerned with many infinite sets, larger and smaller, it is a good idea to learn about infinite sets if one wishes to understand mathematical objects better.

And one can study, naively, a lot of set theory, especially under the tutelage of a good teacher that will actually teach axiomatic set theory in a naive guise. And this sort of learning can, and perhaps should, include discussions about the axiom of choice, about ordinals, and about cardinals. As Ittay said, and I'm agreeing ordinals and cardinals are two ways of counting, which extend beyond our intuitive understand that counting is done via the natural numbers, and allow us to count infinite objects.

If one couples these ideas with the basics of first-order logic, predicate calculus, and basic first-order logic, one understands how set theory can be used as a basis for modern mathematics. Which again, allows us to better see into some parts of mathematics.

Axiomatic set theory, on the other hand, is a mathematical field like any other. It has certain type of typical problems, and set theorists work in their typical or atypical ways to solve them, or at least understand them better. Axiomatic set theory does, however, handle the fine-grained problems that come from infinity better.

Why do I mean by that? A lot of the infinite sets in modern mathematics are countable or have size continuum. Rarely we run into larger sets (e.g. the set of all Lebesgue measurable sets is larger), but even then we rarely care about that. But now that we understand infinite sets better, we can ask questions like "Given an abelian group with such and such properties, is it necessarily free [abelian]?" usually we can prove these sort of theorems for countable objects, in this case countable groups, but not beyond that.

Sometimes we are interested in topology, which allows us to extend our control from countable objects to things that can be approximated "in a good way" with countable objects (like separable spaces). But even then we can ask questions which involved an arbitrary objects, and not necessarily one which has 'nice properties'.

It turns out that our lack of intuition for infinite sets is reflected in the lack of "naively provable structure" of infinite sets. We cannot even provably determine how many distinct cardinalities lie between the cardinality of $\Bbb N$ and $\Bbb R$. It might be none, or it might be one or two or many more. Here axiomatic set theory comes into play.

Axiomatic set theory deals with the additional axioms that we can require the set theoretic universe to have, and how they affect the structure of infinite sets. And this is the importance of set theory to mathematical research, as well. It deals with solving the existence or what sort of assumptions we need to prove, or disprove, the existence of certain objects.

These objects, while seemingly arbitrary, can have a great influence and strong effects on the structure of "mathematically interesting sets". For example, we know that every Borel set is Lebesgue measurable. But the continuous image of a Borel set need not be Borel. Is it Lebesgue measurable? It turns out that yes, but if we close the Borel sets under complements and continuous functions, will the resulting sets be Lebesgue measurable? Will they satisfy some sort of "continuum hypothesis"? Will they have the Baire property? And other questions, which are all quite natural, originated all sort of strange set theoretical objects and axioms which assert their existence.

And if you ask me, that is why we should learn set theory, and what its importance is. It allows us to better understand infinite objects, and the assumptions needed to better control their behavior.

• Would you mind if I also give a link to this paper of yours for future readers?
– user170039
Apr 24, 2016 at 15:34
• No, that's why it's online... Apr 24, 2016 at 21:29
• @AsafKaragila I agree that set theory revolves a lot around infinite sets. How about the finitists then? Is there anything in set theory for them? (Note: I am not a finitist myself. I do however have a bit of a trouble pointing my finger on the specific elements that elevate these foundational subjects beyond a mere mental exercise in beauty and logical structure). Mar 2, 2020 at 8:17
• @Pellenthor: Well, there are different levels of finitism. Some finitists work in arithmetic, and there understanding ordinals is still useful, so understanding set theory is helpful with that respect. Other are "ultrafinitists" and argue that there is a finite universe of mathematics. These people reject even the basic induction and recursion theorems. Set theory is not very helpful there, I suppose. But frankly, this is a fringe population (some of which are not even defining themselves as mathematicians, but rather as physicists/computer scientists/etc.), so I don't know what to say there. Mar 2, 2020 at 8:21
• @Pellenthor: (1) This is definitely a lot more than I would be comfortable discussing in the comments; (2) arguably there is none, because even all the fancy stuff they do in physics that requires infinity can be either (i) circumvented by approximating, or (ii) relies on things which are essentially countable (e.g. separable spaces); (3) if you do wish to pull on this thread, you need to spend some time contemplating philosophy of mathematics and how things influence other things in mathematics, understand interactions between applied math and engineering will also help to make this bridge. Mar 2, 2020 at 9:26

Naive set theory:

Set theory is the common language to speak about mathematics, so learning set theory means learning the common language. Another aspect is that of counting. Cardinality of sets is a very fundamental notion which can be treated naively quite efficiently. Cardinality means counting, so learning set theory means learning to count (beyond the finite numbers). A classical application is the proof of the existence of transcendental numbers. Lastly, set theory is securely tied with logic, so learning set theory means learning logic, which we kind of use all the time.

Axiomatic set theory:

Set theory is so fundamental that the only way to rigorously study it is axiomatically. Moreover, very early on in the naive study of sets one encounters very simple questions which can't be answered. For instance, does every subset of the reals have cardinality of the reals or of the naturals. Another issue is the axiom of choice which on the one hand is equivalent to many obviously true statements, as well as obviously false statements. This situation necessitates a careful axiomatic study, and, of course, there are several different axiomatization giving rise to different theories of sets, making it all more fun.

To expand on Ittay Weiss's post, and maybe this is not exactly what you are looking for, but set theory deals with the mathematical universe. More concrete, set theory has the ability to describe independence results. This, in my opinion, is the real importance in set theory. As Ittay alluded to above, consider $\mathbb{R}$. Does there exist a set $A$ such that $|\mathbb{N}|<|A| < |\mathbb{R}|$? Set theory has shown us that the solution to this question is beyond our intuition (ZFC). It is $possible$ to construct models of the set theoretic universe where this statement is true as well as construct universes where this statement is false. Therefore, we can never know the solution to this question.

Furthermore, the existence of certain combinatorial objects is also independent of our intuition. However, it should also be noted that set theory does not exist in a bubble. Results in set theory bleed over into other areas of mathematics. Consider, for instances, Shelah's solution to Whitehead's problem. Independence results appear throughout mathematics and it is the job of set theory to explain why and how these indepedence results occur.

Remark: I have tried to be careful with my wording and tried not to say anything wrong. However, the philosophy of set theory is an alive field and it would take one years of study to comprehend the arguments of Woodin, Hamkins, and others who have discussed these topics at length. The above is only my humble opinion.

• "the solution to this question is beyond our intuition (ZFC)" — not everyone's intuition is same. Not everyone agrees that Choice is intuitive. In the other direction, one may well consider it intuitive that every set should be constructible (i.e. $V=L$), in which case the intuition suffices to answer that specific question ($V=L$ implies the continuum hypothesis). Jul 27, 2016 at 8:54