In this post I’ll show how Set Theory allows us to define the natural numbers using sets. It’s admittedly a very abstract topic, but it’s about something very common in our experience: counting things. Seeing how numbers are defined also demonstrates (contrary to some false notions) that there is a huge difference between a *number* and how that number is “spelled” or represented.

**Note:** *I am not a mathematician!* This topic is right on the edge of my mathematical frontier. I wanted this addendum to the previous post, but be aware I may misstep. I welcome any feedback from Real Mathematicians!

But go on anyway… keep reading… I dare ya!

We’re used to using numbers to count things. For example, if we’re playing penny-poker, we know how many pennies we have sitting in front of us. We’re also used to comparing things we’ve counted; we know that 27 pennies is more than 22 pennies.

The question is: *how do we know these things?* What allows us to count, one, two, three? Is that the same as 1, 2, 3? What about I, II, III? What tells us that seven (or 7) is greater than three (or 3)? It’s the basic math we learned in school, but where does it come from? What makes it work the way it does? How do we know it works *right*?

It all starts with Set Theory.

[Full disclosure: not all mathematicians use set theory to define the natural numbers; there are other ways to formally define them. Even so, these alternates share the common approach described below of an initial number and a successor function.]

When we define any mathematical system we start with a small number of rules, or *axioms*, that we take as the core truths of the system. These are the “givens” of the system. (As in, “given X, therefore Y.”) Everything else in the system (*theorems*, the “therefores”) must be built from the axioms.

The geometry you learned in high school — Euclidean geometry with the right angles and circles and lines — has at its core a small set of axioms upon which everything else is constructed. (I was always fascinated by how much you could do in geometry with just a straight-edge and a compass.)

In the previous post I wrote about how sets have *members*. And they have a *size*, which is a count of how many members are in the set. Regardless of what kind of members a set has, we can always compare sets based on their size. A set of 10 pennies and a set of 10 elephants are identical (in size).

In fact, when talking about set sizes, we don’t care what the members are. We only care about how many (the set size). Pennies, elephants, family members or whatever, it doesn’t matter.

To create a formal definition of (natural) numbers, we start with a set that has *no* members. We call this the *empty set*. You can think of it as a grocery bag with nothing in it. This empty set is the first axiom. It’s a given, and we equate it with the number “zero.”

Mathematicians use curly braces, {…}, to notate sets. What’s between the braces is what’s in the set. For example, the set of the first three English capital letters is: {A, B, C}. (Commas separate members.)

The empty set is notated as simply: {}. (I’m going to use different colors for the sets to help keep things clear.)

The second axiom is that we define a *successor function*, which is a way, given some *thing*, to generate the next *thing* in line. (In a sense, ‘having children’ is a successor function to generate the next generation in your personal family tree.)

For the natural numbers, we define a successor function to — given some set (number) — generate the next set (number) in line. We do that by saying that the *next* set in line is a set containing all the previous sets.

So far, we only have one set, the axiomatic empty set, which we’re calling “zero.” To create the next set (number), we create a set containing all previous sets, which in this case is only the empty set. That gives us a set that looks like this: {{}}.

Think of it as putting an empty grocery bag inside another grocery bag. Now you have a grocery bag containing a grocery bag. We’ll call this new set “one.”

Now, because we defined the first set, {}, as 0 (“zero”), we could also write the new set (“one”) as: {0}. It might seem odd that “one” sort of looks like a “zero” with curly braces around it, but our successor function says that any given set (number) contains all previous sets (numbers), so the “one” set contains everything *except* the “one” set.

Note an important thing: the set for “one” contains one member!

Let’s keep going. The next set, which we’re going to call “two” needs to contain the previous sets. We’ve defined “zero” and “one”, so one way to show “two” is the set: {0, 1}. (Note that it has two members!) The formal way to show it looks a bit confusing: {{}, {{}}}. Just remember that {}=0, and {{}}=1.

So now that we have {{}, {{}}}=2, we can define “three” as: {{}, {{}}, {{},{{}}}}. We can also write this as: {0, 1, 2}. And, again, note this set has three members.

We can continue using the successor function to create all the natural numbers. The formal way gets hard to follow as the curly braces proliferate, so from now on, we’ll use the way that uses the digits. For example, the number “nine” is notated as: {0, 1, 2, 3, 4, 5, 6, 7, 8}. (Which has nine members.)

[You might wonder if it wouldn’t be easier to just fill each new grocery bag with X number of empty bags. For instance, “three” would be: {{}, {}, {}}. It does create sets with increasing sizes, but breaks the successor rule of “each next set includes all previous sets.” There are some important set-theoretical reasons for that rule.]

A very important point here is that, although we’ve said {}=0, {{}}=1, and so forth, the actual number symbols (1,2,3,…) or their names (“one”, “two”, “three”,…) are not part of the definition! This is crucial to understand! *The definition of the natural numbers is strictly based on set sizes.*

Think of this in terms of grocery bags or piles of pennies. All we care about is that a given set has a given size. These set sizes have a variety of ways they can be expressed, and all such expressions are equal. For example:

{{}} = one = 1 = I = uno = ein = ένας = एक = satu = один

These are all various ways of “spelling” the number of noses most people have on their face. (My apologies to those with a lesser or greater number.)

There is a *world* of difference between a numeric quantity and the way that quantity is notated or “spelled.” This is rather obvious when it comes to different languages (and hopefully Google Translate did me right on the last four above).

This may be less obvious when it comes to the familiar (base ten) notation of digits that most of the world uses for basic math. I’ve actually heard of people who think that our base ten system is somehow “special” or different from other bases.

It’s not. It’s purely coincidence based on the usual number of fingers most people have. Other cultures have, in fact, used other number bases. And when it comes to counting things, no number base is any better than any other (save, maybe, for how many symbols you have to memorize and for the length of the number string).

But I think that’s enough for today. Next time I’ll continue with number bases, and if you stick with me, you’ll walk away understanding “binary” and “octal” and “hexadecimal” number systems.

**Summary**: Every natural number is defined as a set that contains all previous (“less than”) sets (numbers). The size of the set *is* the number! In a very real sense, each number is just a size (formal term: *cardinality*) of the set representing it. Numbers have a variety of — equivalent — ways of notating or “spelling” them.

June 26th, 2014 at 6:27 am

OK. I read part one … all of the way through and was going to make a pithy comment about the beer. Because that I understood. Then I thought, OK, that’s mature.

Was going to pass *this* part by… until you dared me. Dang! I cannot resist a dare!

June 26th, 2014 at 11:49 am

Oh, so

that’sthe key:daremy readers to keep reading! Thanks; I’ll have to keep that in mind! 😀Pithy comments (even long-winded ones) are always welcome. Beer is liquid bread, which makes it the stuff of life! (Remember that, while beer has food value, food has no beer value, so clearly beer is superior. And alcohol kills germs, so beer is medicinal, too!)

All seriousness aside, I don’t figure I can pack everything I need into articles like these to make them completely comprehensible to those unfamiliar with the topic. To some extent I put them out there so I can refer back to them in other places, usually in support of something I’m saying to people who are already (at least somewhat) familiar with the material. Or so that I can refer back to them in future article as I try to build a more complete edifice.

For those who are new to the subject, I can imagine things might not click into place, and I feel like

Star Trek‘s Guardian of Forever waiting for a question.Maybe I should be

daringpeople to ask questions! 🙂Thanks for reading and commenting! Nice talkin’ to ya!

June 26th, 2014 at 1:58 pm

Nice talking to you, too!

Just so you know, I was able to understand your work – and I also want to acknowledge the amount of effort that you put into formatting and explaining in plain language. Would I be able to recount it to someone else? Yeah, no.

I am in year two of a three year tech program. The math stuff is done and behind me. Some of it was beyond brutal for my aging synapses. But I managed. I don’t think I’d be reading your posts, otherwise, dare or not!

Oh man! Now I need to brush up on my math AND Star Trek? When will it end? 😉

June 26th, 2014 at 2:59 pm

Around here

Star Treknever ends! (Despite a certain director’s best efforts to kill it.)It’s really nice to have someone say they were able to understand the work — I really appreciate that! FWIW, I’ve found that trying to explain something to someone can really help one understand it better. I’ve found that here on this blog, and I discovered it earlier in life when I did some teaching and tutoring. That’s why I love questions; they often make me realize I didn’t quite understand something, so sometimes my answer is, “Hmmm… I dunno… let me go try to find out!”

What kind of tech program? Something aligned with minerals or geology?

June 26th, 2014 at 3:52 pm

I know exactly what you mean about learning from teaching. I put together material for a social services course. (That was two lifetimes ago) The effort required to make it understandable for the students secured it in my own noggin.

You are correct! I am studying mining technology. Grads go on to jobs such as surveying, slope stability monitoring, mapping, drilling, ventilation, and dozens of other mining related jobs. I see myself as an AutoCAD tech. Maybe. I’ll let the universe lead the way on this one.

June 26th, 2014 at 5:02 pm

Neat! CADCAM is cool!! I’ve never used AutoCAD, but many years ago I was trained in (very simple use of) Unigraphics CADCAM in order to provide IT support for The Company’s engineering groups. I always thought that was tons of fun, and modern CADCAM seems to take it to a whole new level of, “Oh, that’s amazing!”