So, what’s set theory really about?

We’ll start off, for intuition’s sake, by talking a little bit about what’s now called naive set theory, before moving into the formality of axiomatic set theory. Most of this post might be a bit boring for a lot of you, but it’s worth
being a bit on the pedantic side to make sure that we’re starting from a clear basis.

A set is a collection of things. What it means to be a member of a set S is
that there’s some predicate P_S – that is, some way of describing things via logic – which is true only for members of S. To be a tad more formal, that means that for any possible object x, P_S(x) is true if and only if
x is a member of S. In general, I’ll just write S for both the set and the predicate that defines the set, unless there’s some reason that that would be confusing and/or ambiguous. (Another way of saying that is that a set S is a collection of things that all share
some property, which is the defining property of the set. When you work through
the formality of what a property means, that’s just another way of saying that there’s a
predicate.)

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While I’ve been writing about the Surreal numbers lately, it reminded me of some of the fun of Set theory. As a result, I’ve been going back to look at some old books. Since I’ve been enjoying it, I thought you folks would as well.

Set theory, along with its cousin, first order predicate logic, is pretty much the
foundation of nearly all modern math. You can construct math from a lot of
different foundations, but axiomatic set theory is currently pretty much the dominant approach. (Although Topoi seem to be making some headway…)

There’s a reason for that. Set theory starts with some of the simplest ideas, and extends in a reasonably straightforward way to encompass the most astonishingly complicated one. It’s truly remarkable in that – none of the competitors to set theory
as a foundation can approach the intuitive simplicity of set theory.

So I’m going to write a bit about set theory as I explore my old books. And I thought that a good place to start was Cantor’s diagonalization. Cantor is the inventor of set theory, and the diagonalization is an example of one of the first major results that Cantor published. It’s also a good excuse for talking a little bit about where set theory came from, which is not what most people expect. Set theory was originally created as a tool for talking about the relative sizes of different infinities.

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