Most of the shocking results in mathematical logic have to do with how limited we are in terms of producing good deductive systems for mathematical concepts. (Good here means sound, complete, finitary, and computable.) No logic with a good deductive system can categorically define the natural numbers. Adding a “for finitely many” quantifier to first-order logic makes it impossible to have a good deductive system. Ditto for a “for all subsets of” quantifier. And so on.

Once you’ve accustomed yourself to the extreme logical limitations imposed on us, it comes as a bit of a shock when you realize that we still have the ability to describe ANY complicated mathematical concept. What I learned recently was that you can extend first-order logic by adding a “for uncountably many” quantifier, and still have a good deductive system for the logic! It says something that I was so impressed by this fairly moderate ability to distinguish between the countable and the uncountable. But it certainly does feel puzzling that while you can distinguish between the countable and the uncountable, you can’t distinguish between the finite and the infinite, which seems like a much more basic and fundamental division!

What’s even cooler is that the deductive system for this extended logic is extremely simple. It amounts to taking the axioms and inference rules of first-order logic, and then tacking on four additional axiom schemas. We’ll write “for uncountably many *x*, *Φ(x)*” as *Ux Φ(x)*.

*∀y∀z¬Ux (x = y ∨ x = z)**∀x (Φ(x) → Ψ(x)) → (Ux Φ(x) → Ux Ψ(x))**Ux Φ(x) ↔ Uy Φ(y)*, where y isn’t free in*Φ(x)**Uy∃x Φ(x,y) → (∃xUy Φ(x,y) ∨ Ux∃y Φ(x,y))*

Each of these axioms schemas is conceptually quite easy to make sense of. The first says that for any two elements y and z, there’s not uncountably many things that are each equal to one or the other. In other words, uncountable is bigger than two.

The second says that if the set of things that satisfy *Φ* is a subset of the set of things that satisfy *Ψ*, and uncountably many things satisfy *Φ*, then uncountably many things satisfy *Ψ*. In other words, if a set has an uncountable subset, then it must be uncountable itself.

The third is obvious enough that it doesn’t need explanation. The fourth, on the other hand, does. I find it helpful to think of *Φ(x,y)* as saying “x points at y.” Then *Uy∃x Φ(x,y) *becomes “uncountably many things are pointed at”, *∃xUy Φ(x,y)* becomes “some person points at uncountably many things”, and *Ux∃y Φ(x,y)* becomes “there are uncountably many people pointing.”

So the fourth axiom just says that if uncountably many things are pointed at, then either somebody is pointing at uncountably many things, or there are uncountably many people pointing at things. Otherwise you’d only have countably many people pointing at countably many things each, and that’s not enough to get uncountably many things pointed at! So this fourth axiom can really be understood as saying that the union of countably many countable sets is countable.

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There’s a caveat to the above claims. This is that first-order logic extended by the “for uncountably many” quantifier only has a good deductive system IF the language is restricted to only allow countably many constant symbols. Here’s the reasoning:

First of all, if a logic has a sound, complete and finitary deductive system, then it is compact. Discussed here.

Second, if a logic L is compact and a set of sentences Σ in L has a countably infinite model, then Σ also has an uncountable model. Why? Simply append to L an uncountable set A of constant symbols, and add to Σ an uncountable set of sentences declaring that each of these constants refer to a distinct object. Now we have an uncountable model of Σ’s extension by compactness (the countably infinite model is a model of every finite subset of Σ’s extension), and this uncountable model must also satisfy Σ. (This is the only place where we make use of uncountably many constant symbols.)

Third, if a logic L is compact, and a set of sentences Σ in L has models of arbitrarily large finite size, then Σ has infinite models. The proof of this is virtually identical to the last paragraph; add infinitely many constant symbols and declare them all to refer to distinct objects. Every finite subset of this extension of Σ has a model, so by compactness the extension of Σ also has a model. This model is infinite by construction, and as a model of an extension of Σ must also be a model of Σ.

That sets up all the background model theory we need. Now, suppose that FOL+U had a good deductive system that applied even with uncountably many constant symbols. By soundness, completeness, and finiteness, FOL + U would be compact. Consider now the FOL+U theory T consisting of the single axiom *¬Ux (x = x)*. T cannot have uncountable models, by assumption that the quantifier *Ux* is really capturing the semantics of “for uncountably many x”.

But if T can’t have uncountable models, then it can’t have a countably infinite models either, by the second result above! So T doesn’t have any infinite models. And then by the third result, T cannot have models of arbitrarily large finite size! This means that asserting “there aren’t uncountably many objects”, we actually end up ruling out all countably infinite models, as well as all models larger than some finite size! This is inconsistent with the claim that the *U* quantifier is correctly capturing the semantics of “uncountably many”; after all, the negation of “for uncountably many” should be “for countably many”, not “for small enough finite”!

Summing up the argument:

- If a logic has a sound, complete, and finitary deductive system, then it is compact.
- If a logic L is compact, and a set of sentences Σ in L has a countable model, then Σ has an uncountable model.
- If a logic L is compact, and a set of sentences Σ in L has models of arbitrary finite size, then Σ has infinite models.
- FOL+U = First order logic + “for uncountably many” is sound, complete, and finitary.
- The FOL+U theory T = {
*¬Ux (x = x)*} doesn’t have uncountable models. - By 1 and 4, FOL+U is compact.
- By 2 and 6, if a set of sentences Σ in FOL+U has a countable model, then Σ has an uncountable model.
- Suppose T had countably infinite models. Then by 7, it would also have to have uncountable models, contradicting 5. So T doesn’t have countably infinite models.
- Suppose T had arbitrarily large finite models. Then by 3 and 6, T would have infinite models, contradicting 5 and 8. So T doesn’t have arbitrarily large finite models.
- Rephrasing 9, there’s some finite n such that T has no models of cardinality larger than n.

✯✯✯

What’s great about the logic FOL+U is that it allows you to talk about the property of uncountability, so long as your theories are all only countably large (which isn’t too bad a restriction). So for instance, consider the FOL+U theory consisting of the axioms of first-order Peano arithmetic plus one additional axiom: *¬Ux (x = x)*. This theory successfully rules out all uncountable models! This is something that you couldn’t do in ordinary first order logic, since it obeys the upward Lowenheim-Skolem property (a model of one infinite cardinality implies models of every larger infinite cardinality). The order type of all countable nonstandard models of PA is known (each is ω + ℤ⋅ℚ, a natural number line followed by countably many integer lines arranged like the rationals), and much of the difficulty of the model theory of PA involves the uncountable models. So this extension of PA is a big improvement over first-order PA!

Remember that ZFC has those pesky countable models, in which “the set of all subsets” turns out to be missing some subsets? We can get rid of all of those countable models with the FOL+U theory consisting of the axioms of ZFC + the additional axiom *Ux (x = x)*. I’m pretty sure that this does not fix all the issues with correctly pinning down the notion of the power set — that would be too good to be true — but it is certainly an improvement!

Where did you read up on U, I’ve never heard of it.

There’s also a small fraction of people who argue that SOL isn’t all that bad. Something, something Heyting models. I mean I can see that some might say SOL is only roughly as bad as set theory. Personally the empty set gives me a few headaches and just having to deal with the always false proposition sounds like a good deal, even if some metalogical properties go out of the window.

Paper on FOL+U (U is more standardly written as Q, and FOL+U is more standardly written as L(Q)): https://www.sciencedirect.com/science/article/pii/S0003484370800055

I do go back and forth a lot on how I feel about second-order logic. On the one hand, it allows us to categorically define all sorts of useful concepts like “the natural numbers” and “the set of all subsets”, but on the other hand I’m not sure how much that’s worth when we don’t have any good deductive system that allows us to calculate the logical consequences of these concepts.

(Also, even if I was totally okay with second-order logic I’d still be independently interested in the limits of what we can define using logics with sound, complete, and computable deductive systems.)