Axiom of Choice

Definition of Axiom of Choice

The Axiom of Choice (AC) sounds like a fancy math thing, but it’s really just about making choices. Imagine you have a huge collection of boxes. Each box has some marbles inside, and no box is empty. The AC says that you can pick one marble from each box, even if you have an endless number of boxes and no way to tell which marble to pick from each one. In other words, for any group of non-empty groups (sets), there’s at least one set – we call this a “choice set” – that has exactly one item from all the other sets.

To put it another way, think of a huge library with an endless number of books. Each book is a set filled with stories, and the library is a set of books. The Axiom of Choice lets you create a new book by selecting just one story from each book. You don’t need rules for picking the stories; you just know you can do it. This new book is your “choice set” with a story from every book in the library.


Even though the Axiom of Choice is unique, it does have variations for special situations. Think of it like having different tools for different tasks – while they all do similar jobs, some are just better suited for certain tasks.

  • The “Principle of Dependent Choices” can be thought of like making a chain of choices where each choice depends on the previous one.
  • The “Axiom of Countable Choice” is for when you’re only picking from a list of sets that you can count, like 1st set, 2nd set, and so on.

Examples of Axiom of Choice

  • Socks and Shoes:

    Let’s say you have an endless amount of socks and shoes pairs. Choosing a sock from each pair needs AC because socks in a pair look the same – there’s no easy rule to pick them. But for shoes, you can choose the left one every time, hence AC is not necessary. This is an example of AC because it shows the difference when you don’t have a simple way to choose.

  • Infinite Product of Non-empty Sets:

    Imagine endless sets, each with at least one number. AC lets you choose one number from each to make a new set. It’s an example because without AC, you couldn’t make this set if you tried to choose from infinitely many sets.

Why is it important?

The Axiom of Choice is a crucial math tool that lets us say “yes” to problems we can’t otherwise solve. It’s like having a magic key that opens any lock in a giant building. With AC, we have the power to pick an infinite number of choices all at once, even if there’s no clear way to pick. This is super useful in all different parts of math. If you’re not a mathematician, think of it like a basic rule that makes a lot of advanced technology and science possible – it’s behind-the-scenes but super important.

Implications and Applications

The reach of the Axiom of Choice is wide, touching almost every field of math. One example is in proving that any space with directions (vector space) has a “basis,” meaning you can describe every point in that space a certain way. It also helps in sorting (well-ordering) sets in a line where every group has a “smallest” item. This isn’t easy to see for all sets, like numbers, but AC makes it work.

Comparison with Related Axioms

When we talk about the AC, we often hear it with two other math principles: the Zermelo–Fraenkel set theory (ZF) and the Generalized Continuum Hypothesis (GCH). All together, they form ZFC set theory, which is the base for a lot of math out there. AC stands on its own in ZF; you can’t prove it right or wrong using the other ZF principles.


AC started in the 1900s with a math guy named Ernst Zermelo. He brought it up to prove a certain order could be made for sets. It was a big deal, and AC began to matter a lot in math from that point.


Some math people are all in for AC because it’s like a puzzle solver. Others aren’t so sure because it can make weird things seem possible, like the “Banach-Tarski Paradox” – imagine splitting a ball into pieces then putting it back into not one, but two whole balls the same size. This can’t happen in real life, but AC says it’s ok in math.

Related Topics

  • Zermelo-Fraenkel Set Theory (ZF): This is like the rulebook for a type of math called set theory. It gives a set of rules, but doesn’t include the Axiom of Choice.
  • Generalized Continuum Hypothesis (GCH): This is a statement about the sizes of infinite sets, a topic that’s made clearer with tools like the Axiom of Choice.
  • Banach-Tarski Paradox: A weird result that comes from AC, telling us about dividing and recreating shapes in an impossible way.


So, the Axiom of Choice is a big idea in set theory, which is a part of math. It’s all about picking an item from a bunch of sets, even when there’s no end to the sets or clear way to pick. It’s made a difference in many parts of math and started some intense chats among math folks. It might not change what you do every day, but in the math world, it’s got a major role, and knowing about it helps us see how math brains think about numbers, shapes, and more strange ideas.