The subtlety of Gödel’s second incompleteness theorem

Gödel’s second incompleteness theorem is an order of magnitude more subtle than his first. It’s commonly summarized as “no consistent theory strong enough to do arithmetic can prove its own consistency.” But there’s a lot of subtlety in both the “strong enough to do arithmetic” and the “prove its own consistency.” First of all, what exactly counts as strong enough to do arithmetic? Peano arithmetic certainly does, but it has an infinite axiom schema. Do any finite theories meet the criterion of “strong enough to do arithmetic”? It urns out that the answer to this is yes! Robinson arithmetic, which is what you get if you remove the infinite axiom schema of induction from PA, meets the requirement.

There are weaker theories of arithmetic, like Presburger arithmetic (which has only addition) and Skolem arithmetic (only multiplication), that don’t meet the criterion, and are therefore not subject to the incompleteness theorems. And it turns out that both of these theories are actually decidable! This is even stronger than being sound and complete; soundness and completeness tell us that there’s an algorithm that determines after a finite amount of time that a given sentence is a tautology, but not necessarily that such an algorithm exists to determine that a sentence is NOT a tautology. Decidability gives us both: not only can we classify any tautology as such in a finite time, we can also classify any non-tautology as such in a finite time.

The amount of arithmetic we need is exactly the amount that Gödel uses to prove the incompleteness theorems. This has two parts: one, the theory must express enough arithmetic to do Gödel encoding (and therefore to express the notion of “provability”), and two, the theory must be able to formalize diagonalization. Dan Willard came up with theories that formalize enough arithmetic to do the first but not the second: these are theories that can talk about their own provability via Gödel coding, but are still too weak to be subject to the incompleteness theorems. Thus, these theories can actually prove their own consistency! These fascinating theories are called self-verifying theories.

Everything I’ve said so far has been about the subtlety of the notion of “strong enough to do arithmetic”. Now I want to talk about the subtlety of the notion of “proving one’s own consistency.” I’ll do this by first taking a brief interlude to talk about an argument I recently saw against the Consistency Thesis in philosophy of math that uses this notion. A friend of mine writes:

Consistency is a weak soundness property.

Here’s what I’ll call the Consistency Thesis: (Mathematical sentences are justified and/or true when they are part of a consistent theory.) What I want to do is to raise a problem for that thesis. Maintaining the Consistency Thesis leads to bizarre mathematical beliefs which don’t lend themselves to being true or justified. A merely ‘consistent’ theory can claim P(0), P(1), and so on, and yet claim that there’s some n for which ¬P(n). Such a theory is omega-inconsistent.

There are mathematical theories which despite their syntactic consistency, claim to be inconsistent. They will claim to be able to derive that 0=1 and yet never derive 0=1. One example of a consistent but unsound theory would be this: [suppose we add to ZFC the new axiom “ZFC is inconsistent”. If ZFC is consistent, then the new theory ZFC’ is consistent (by Godel’s second incompleteness theorem), even though it falsely proves that ZFC is inconsistent.]

So far, I’ve only mentioned omega-inconsistent theories. I should also mention that there are omega-consistent theories which are arithmetically unsound. A much stronger soundness property would be soundness in omega-logic, which entails consistency, omega-consistency, and arithmetical soundness.

Here’s the response I gave:

You say that an adherent to the Consistency Thesis believes in mathematical theories that are in fact consistent (no contradictions can be proved from them) but claim to be inconsistent; for instance ZFC + ¬Con(ZFC). This bit about “claiming to be inconsistent” is subtler than it might initially seem. There’s a very important difference between Con(ZFC) and “ZFC is consistent”. A first order theory can’t talk directly about its own consistency, it can only talk about properties of the objects in its models. We are allowed an indirect method to talk about consistency of theories, Gödel encoding. But this method has problems.

Gödel encoding allows us to write down statements that, if understood to be about the natural numbers, are equivalent to the assertion that a theory proves a contradiction. But this “if understood to be about the natural numbers” is a very important qualification, because no first order theory categorically defines the natural numbers (i.e. no first order theory has as its only model the natural numbers). More generally, no theory within a logic that has a sound, complete, and finitary proof system categorically describes the natural numbers (these are the only logics that a Formalist will see as well-defined, by the way).

What this means is that when we write “Con(ZFC)”, we’re actually using a short-hand for a complicated sentence about the objects in our models, and this complicated sentence is NOT equivalent to the claim that no contradiction can be proven from ZFC. Con(ZFC) could be false in a model even if ZFC is consistent, and Con(ZFC) could be true in a model even if ZFC is inconsistent, so long as that model is not the standard natural numbers.

So the adherent of the Consistency Thesis is not actually committed to weird beliefs about a theory being consistent and claiming its own consistency; they are just committed to the belief that the natural numbers are not well-defined.

The same objection applies to the claim that they have to accept as valid theories that claim P(0), P(1), and so on, but also that there’s some n for which ¬P(n). That’s true, and that’s fine! One can just say that such theories are not theories of the standard natural numbers; the n for which ¬P(n) is some other type of mathematical object that is not a natural number.

A TL;DR for my response: “Con(T)” only means “T is consistent” if T is about the natural numbers. Furthermore, the theories that assert their own inconsistency never have the natural numbers as a model. So it’s ultimately not very weird that these theories assert “¬Con(T)”… this statement doesn’t actually mean “T is inconsistent” in any of the models of the theory!