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In his Mathematical Analysis I, Zorich says the following after introducing the reals axiomatically:

In relation to any abstract system of axioms, at least two questions arise immedi- ately. First, are these axioms consistent? That is, does there exist a set satisfying all the conditions just listed? This is the problem of consistency of the axioms. Second, does the given system of axioms determine the mathematical object uniquely? That is, as the logicians would say, is the axiom system categorical? Here uniqueness must be understood as follows...

Now I am no logician or philosopher so I certainly don't expect, want, or need the full treatment, but I am wondering if it is at all "controversial" to assume that consistency implies existence in the sense which seems to be tacit in Zorich passing from

First, are these axioms consistent?

to using "that is" in

That is, does there exist a set satisfying all the conditions just listed?

EE18
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  • In a precise sense this is justified: https://en.wikipedia.org/wiki/G%C3%B6del%27s_completeness_theorem – Noah Schweber Feb 22 '24 at 20:11
  • Having only heard but knowing nothing of his (incompleteness) theorems, I did not expect an answer to be found therein. Thanks for the info! @NoahSchweber – EE18 Feb 22 '24 at 20:12
  • Just to be sure, I should take the refrain "anything true in all models is provable" to be an affirmative answer to my question? @NoahSchweber I'd be happy to accept a simple answer to that effect. – EE18 Feb 22 '24 at 20:14

1 Answers1

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Godel's perhaps-unfortunately-named completeness theorem gives a precise positive answer: if $T$ is a set of (first-order) sentences and every model of $T$ satisfies the (first-order) sentence $\varphi$, then in fact there is a proof of $\varphi$ from $T$. More snappily, we have $$T\vdash\varphi\iff T\models\varphi$$ (technically completeness is the right-to-left direction, with the left-to-right direction being soundness, but soundness is so trivial it's often subsumed by completeness).

In particular, let $T$ be "any abstract system of axioms" (as long as they're first-order) and let $\varphi$ be $\perp$, the always-false sentence. Then "$T\models\varphi$" means "Every model of $T$ satisfies the always-false sentence," which is another way of saying "$T$ is unsatisfiable;" meanwhile, "$T\vdash\varphi$" is just another way of saying "$T$ is consistent." Contrapositing, we get that if $T$ is consistent then $T$ is satisfiable (= has a model).

(Note that "consistent iff satisfiable" is equivalent to the completeness theorem as stated above, since we can shift from "$T\not\vdash\varphi$" to "$T\cup\{\neg\varphi\}$ is consistent" and similarly for $\models$. However, this trick doesn't work in logics without negation, so in general there is a real difference here. But this is a side issue.)

There are two crucial caveats here:

  • It is absolutely essential that we stick to first-order sentences. In general, logics admitting a completeness theorem are quite rare.

  • It is also absolutely general that we look at all models of $T$ in the definition of $T\models\varphi$. The common phrasing of Godel's incompleteness theorem as "There are true statements of arithmetic which aren't provable" may appear to contradict the completeness theorem, but of course it doesn't: the issue is that "true" here refers to truth in one particular model, and that's not something that the completeness theorem has bearing on.

Noah Schweber
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  • Thanks very much for this answer. Just before I accept, would it be possible to explain what $T$ and $\varphi$ are in my case here? My facility with the requisite mathematical logic is unfortunately limited. – EE18 Feb 22 '24 at 21:49
  • @EE18 I've edited to address this. Briefly, $T$ is your "any abstract system of axioms" and $\varphi$ is just $\perp$, the always-false sentence. – Noah Schweber Feb 22 '24 at 22:42
  • Super, thank you so much! – EE18 Feb 22 '24 at 23:12