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I've seen it written in a number of places that one of the "advantages" of NBG over ZFC is that it is finitely axiomatizable.

I was wondering, what are some examples of how this is advantageous?

I see no philosophical advantage of finite, as opposed to recursively enumerable, axiomitization since it's always seemed the important thing is being to effectively decide whether or not a statement is an axiom. Plus, there are usually an infinite number of logical axioms around anyway, though I suppose this isn't essential.

So I guess I'm interested in practical advantages. On the other hand, I haven't thought very deeply about possible philosophical advantages, so if I'm missing something there, feel free to discuss that as well.

  • Do you remember any references where this is mentioned? I've never thought of it that way. – Andrés E. Caicedo Feb 09 '18 at 03:50
  • @AndrésE.Caicedo I don't recall any formal references. This was prompted cause Andreas Blass said something to this effect in a comment on here that I read a few days ago (though I can't find it now). It's one of those things that I hear a lot in passing, for instance see Andre and Henning on this question https://math.stackexchange.com/questions/136215/difference-between-zfc-nbg – spaceisdarkgreen Feb 09 '18 at 04:02
  • In the same way that hand tools have advantages over power tools? – Dan Christensen Feb 09 '18 at 04:18

2 Answers2

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Here is a silly thing; I am not sure it is an "advantage" (or, for that matter, a disadvantage), but it indicates a difference:

Inside a model $M$ of $\mathsf{ZF}$ there may be "hidden'' models $N$ of $\mathsf{ZF}$. The situation I have in mind is something like the following, which uses the fact that $\mathsf{ZF}$ is not finitely axiomatizable: $\omega^M$ is nonstandard, $n\in M$ is an infinite natural number, $N\in M$, $M\models E\subseteq N\times N$, and $M$ does not think that $(N,E)\models\mathsf{ZF}$ because it fails to satisfy, say, replacement for $\Sigma_n$ formulas, and $n$ is smallest possible. In this situation, $(N,E)$ is actually a model of $\mathsf{ZF}$, but $M$ does not know it.

(Or, rather than $(N,E)$, the structure $(N^*,E^*)$, where $N^*=\{a\in M : M\models a\in N\}$ and, for $a,b\in N^*$, $a\mathrel{E^*}b$ if and only if $M\models a\mathrel{E}b$.)

This situation actually happens, since by the reflection theorem, any finite fragment of $\mathsf{ZF}$ has models, but $M$ could be a model of $\lnot\mathrm{Con}(\mathsf{ZF})$. So, given such an $M$ (which, necessarily, has a nonstandard $\omega$), the finite (from the point of view of $M$) fragment of $\mathsf{ZF}$ where for some infinite (from the outside) $k\in\omega^M$ we only consider replacement for $\Sigma_m$ formulas with $m<k$ has a model $(N,E)$ in $M$. Since $M$ believes that $\mathsf{ZF}$ is inconsistent, there must be a least $n$ (which, necessarily, is infinite from the outside) such that $(N,E)$ does not satisfy replacement for $\Sigma_n$ formulas.

On the other hand, there can be no such "hidden" models of $\mathsf{NBG}$, by finite axiomatizability.

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    In different terminology, this means that if $T$ is finitely axiomatizable then "$M \models T$" is absolute between models that contain the same set $M$. – Carl Mummert Feb 09 '18 at 14:16
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Philosophically, one could argue that having a finite set of axioms is a more concrete situation than having an infinite but decidable set of axioms. Depending on taste, having a finite set of axioms may be seen as more elegant, or as conceptually simpler. It may help to think of the fact that a finite set of axioms can be replaced with just one axiom, the conjunction of the finite set. So the real difference is between having just one axiom, or infinitely many.

Carl Mummert
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    "There can be only one". That's the Highlander Axiom, which incidentally, is also the only one axiom in the Highlander theory. Thus making the Highlander Theory itself a Highlander theory. – Asaf Karagila Feb 09 '18 at 14:14