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I'm not a mathematician but recently came across Godel's Incompleteness theorems. I have read some of the dumbed down online versions explaining the broad structure of the proof and found the argument quite fascinating. I'm sure the answer to my question is probably explained away in an undergraduate course in logic, however I have searched online for days and can't seem to find the answer - I'm probably not framing the question or search properly.

I do understand that the interpretation/meaning of a logical statement may change if the formal system changes. However I was wondering if it would be possible to gently massage the axioms/rules of the formal system to prove a particular sequence of symbols, that comprise of any logical statement?

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In the comments to Rob Arthan's answer, the OP elaborated on their question:

Is there a similar Forcing type technique (that mostly keeps the interpretation of symbols same) that generates models of formal systems such that a particular statement is true in one model and the negation true in another model while maintaining internal consistency of both models?

It's this clarification which I want to address.

It's a little unclear - in particular, the phrase "that mostly keeps the interpretation of symbols same" doesn't really make sense to me - but I interpret the whole comment as asking:

Suppose $T$ is a theory and $\varphi$ is a sentence independent of $T$. Is there a method for building models of $T\cup\{\varphi\}$ and $T\cup\{\neg\varphi\}$? In particular, is there one which is similar to forcing, in that you start with a model of $T$ and "extend" it in certain ways? And finally, are there further similarities between that method and forcing?

Note that since this is a question about building models as opposed to the consistency of theories, there's a bit of extra subtlety here. (And that aspect is completely missing from the title and body question, so if this is what you're asking you should definitely edit those, and if it's not what you're asking then you need to clarify your question further.)

The answer to your question, interpreted in this way, is yes. Before I leap into the details, let me give a tl;dr: you should look into Goedel's completeness theorem (that's not a typo!) and its proof via Henkinization.


OK, now (some) details:

First of all, let's forget the "base structure" and just look at the process of building a model of a consistent theory. It turns out that this can always be done, and in fact there is a specific method for doing this, called "Henkinization"! While not equivalent to forcing, it (and more advanced ideas along the same lines) do share a family resemblance to forcing: roughly speaking, they build the desired object in "stages," meeting various "requirements" along the way, and relying on a sufficiently "generic" behavior to ensure that they work as desired. (Pinning down what all this means takes a lot of work, and forcing in particular is really really hard; but you might be interested in these takes on forcing from an intuitive perspective.)

So Henkinization lets us explicitly build a model $M$ of any consistent theory $T$. In particular, if $\varphi$ is independent of $T$ then both $T\cup\{\varphi\}$ and $T\cup\{\neg\varphi\}$ are consistent, so we have a method (Henkinization) which can build models of each. Now, what if we try to extend a given structure, rather than build one ex nihilo? That is, suppose $T$ is some theory and $\varphi$ is independent of $T$; and $M$ is a model of $T$. Can I produce models $M_0$, $M_1$ which are "bigger" than $M$, in which $\varphi$ is true and false respectively? (In particular, this process should be guaranteed to work for all $M$ - it shouldn't rely on $M$ having some specific form.)

The answer depends on exactly what you mean by "bigger" (there are lots of ways to compare mathematical structures), but under one interpretation the answer is yes for a wide class of theories. Namely, under pretty reasonable hypotheses about $T$ (it's enough for $T$ to be complete for quantifier-free sentences, which amounts to $T$ deciding exactly what kinds of "local" behavior occur in its models - e.g. the theory of groups isn't complete for quantifier-free sentences, because some groups are non-abelian (= they have the configuration "$a*b\not=b*a$") while others are abelian (= they don't have that configuration)), each of the theories $$T_0=T\cup AtDiag(M)\cup\{\varphi\}$$ and $$T_1=T\cup AtDiag(M)\cup\{\neg\varphi\}$$ is consistent; here "$AtDiag(M)$" is the atomic diagram of $M$, which describes at a very basic level how elements of $M$ relate to each other. Applying Henkinization yields models $M_0$ and $M_1$ of $T_0$ and $T_1$, respectively; and since each of $T_0$ and $T_1$ satisfy the atomic diagram of $M$, $M$ "embeds" in a precise sense into $M_0$ and $M_1$.


EDIT: One can reasonably ask:

How complicated is Henkinization?

This is a bit of a vague question, but one way to make it precise is via computability theory: we can ask, given a computable theory $T$, how easy is it to compute (= build completely explicitly) a model of $T$ by Henkinization? And, how easy is it to compute a model of $T$ via any method (that is, is Henkinization optimal)?

It turns out that Henkinization is optimal, and produces models in an "almost computable" way. Specifically, we can always find a model of $T$ which is low - that is, whose halting problem is no more complicated than the classical halting problem. More precisely, any PA degree computes a model of $T$ (and there are low PA degrees). Such a model can be built via Henkinization, and this result is optimal - there are computable theories, every model of which computes a PA degree.

And in fact, if $T$ is decidable, then the Henkinization process can be made computable! So what complexity there is comes from the technical step of completing the theory, not the model-building part of Henkinization itself. So the takeaway to my mind is that Goedel's completeness theorem is very very close to constructively true.

Noah Schweber
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  • I'd like to know about the downvote to my answer too. However, I don't think our curiosity will ever be satisfied: giving all users the power to deprecate the contribution of others anonymously and with no explanation seems to be enshrined as an indisputable good in the stack exchange ethos. I have tried to do my bit to counterattack in this case by upvoting your and usr21820's answers. – Rob Arthan Apr 15 '17 at 20:21
  • @NoahSchweber: I think you have hit the nail on the head. I need a little time to digest and do more reading up on your answer - will start to rephrase my question subsequently. Just to clarify regarding "that mostly keeps the interpretation of symbols same", I was trying to avoid the situation in Rob Arthan's answer where interpretation of equal symbol completely changed. Godel's Completeness theorems seem way more counter intuitive to me than the Incompleteness theorems, as they/you seem to suggest that any statement can be proven within a carefully constructed model. – Aliquid Ex Nihilo Apr 15 '17 at 20:54
  • @AliquidExNihilo "Godel's Completeness theorem [there's only one] seems way more counter intuitive to me than the Incompleteness theorems" Emphatically yes. I think the completeness theorem is one of the most counterintuitive results in logic, and we often forget how surprising it is. Also, see my edit re: the Henkinization process: we can even show that this construction is "simple" in a precise sense! – Noah Schweber Apr 15 '17 at 21:04
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    Although I slightly quibble with your interpretation of the completeness theorem - I would say satisfied within, rather than proved within. The point is that we don't know if $\varphi$ is always true, or true in the "intended model" if there is one of those; we just know that it is sometimes true, or better yet possibly true given what we know (what we know = our theory). – Noah Schweber Apr 15 '17 at 21:13
  • Finally, as a step towards understanding completeness, it may be easier to start with compactness. This is often proved as a corollary to completeness, but doesn't need to be; and in my mind it should come first. Compactness doesn't talk about proofs, but only about model existence: it says that if $T$ is a theory such that every finite subset of $T$ has a model, then $T$ itself has a model. The connection to provability is this: any proof in $T$ can only use finitely many axioms in $T$, so is really a proof from a finite subset of $T$. (cont'd) – Noah Schweber Apr 15 '17 at 21:15
  • The compactness theorem is still really surprising and hard to fully grasp (in my opinion), but it doesn't mix semantic notions (models) and syntactic notions (proofs) in the same way that completeness does. So you might find it a nice stepping stone towards really "getting" the completeness theorem. – Noah Schweber Apr 15 '17 at 21:16
  • @RobArthan: Thanks for your upvote. In my time on Math SE, I too have gotten a few downvotes on mathematically correct posts with no comments, not just here. I usually ask for my errors to be pointed out so that I can fix them, but it's mostly futile; I never got any response as far as I recall... – user21820 Apr 16 '17 at 16:40
  • @NoahSchweber: Can Henkinization apply to any statement that is not independent of the theory? If yes, then in your comment regarding my interpretation of the completeness theorem, would you still state satisfied within instead of proved within? – Aliquid Ex Nihilo Apr 17 '17 at 03:11
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The question in your title doesn't have much to do with the subtleties of the incompleteness theorems: you cannot prove $x = x \Rightarrow x \neq x$ in any consistent first-order logic. A change to first-order logic that allowed this (e.g., by decreeing that $x = x$ is always false) would probably not be considered as an acceptable interpretation of equality. If you allow such things, a consistent formal system can be designed to make a specific sentence provable, e.g., by giving it as an axiom but disallowing any inferences from that axiom. (The notion of a formal system is a very general one.)

Rob Arthan
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  • Thanks for reading my question literally & carefully. I probably need to reframe the question and perhaps you can help me. Since my original post, I have done a lot more reading and didn't realize the flexibility of formal systems as pointed out by you. I came across the following talk by Joel David Hamkins: https://youtu.be/WndanxPlDFk where he mentions using Forcing to come up with different set-theoretic world's with CH and its negation. Most of the talk was beyond me and I barely understand Forcing - reading through http://www-math.mit.edu/~tchow/mathstuff/forcingdum. – Aliquid Ex Nihilo Apr 12 '17 at 09:06
  • I guess what I'm trying to now ask is: if there are different models of set theory where the interpretation of symbols comprising of the CH statement largely stays the same such that CH is true in one model and it's negation is true in the other; then is there a similar Forcing type technique (that mostly keeps the interpretation of symbols same) that generates models of formal systems such that a particular statement is true in one model and the negation true in another model while maintaining internal consistentcy of both models? – Aliquid Ex Nihilo Apr 12 '17 at 09:19
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    The question in your comment is much clearer. In any first-order theory, $T$, if $\phi$ is a sentence such that $T$ does not prove $\phi$ and does not prove $\lnot\phi$, then you can consistently extend $T$ with either $\phi$ or $\lnot\phi$. The extensions will both have models by the completeness theorem. An example would be $T = PA$ and $\phi$ a sentence asserting that $T$ is consistent. Then if you add $\phi$, $\Bbb{N}$ is still a model, but a model of $\lnot\phi$ will be a non-standard model. – Rob Arthan Apr 12 '17 at 15:45
  • @AliquidExNihilo: Did you even read my post? If you look at the linked computability-based proof, it defines a very general notion of formal systems that is even more general than the kind Rob is talking about. Secondly, if your question is about CH, ask a new question. Don't keep changing the goalposts. – user21820 Apr 13 '17 at 03:04
  • @user21820 I don't fully see the relevance of your answer to the question. And as for CH, the OP's comment is merely using that as an example: they frame their main question as "then is there a similar Forcing type technique (that mostly keeps the interpretation of symbols same) that generates models of formal systems such that a particular statement is true in one model and the negation true in another model while maintaining internal consistentcy of both models?", which is indeed a general statement about (un)provability. It could definitely be clearer but I disagree with your objections. – Noah Schweber Apr 14 '17 at 18:17
  • @user21820: I did read your post, you are again misunderstanding my comment to your answer - in it I actually agree with your assertion that 'there is no such thing as absolute arithmetic truth'. One doesn't need Godel for philosophical musings like absolute truths or self-consistency-proving systems, good old intuition is sufficient for that (though I'm now learning intuition can be misleading especially without well grounded framework); otherwise you are getting something from nothing. Also as pointed out by Noah Schweber my question is not about CH either. – Aliquid Ex Nihilo Apr 15 '17 at 20:40
  • Wait, why on earth did this answer get a downvote? – Noah Schweber Apr 15 '17 at 21:01
  • @user21820: Also, if the answer to my question (as phrased) is in the affirmative then one could find the model that proves sentences negating or skirting Godel's theorems. However that is not thenough motivation for my question, I'm looking for general assertion as explained in Noah Schweber answer. – Aliquid Ex Nihilo Apr 15 '17 at 21:11
  • @AliquidExNihilo: As Noah already pointed out to you, you do not grasp the distinction between semantic truth and syntactic proof. A model does not prove anything whatsoever. Any sentence over the same language will either be true or false in the (first-order) model from the perspective of the meta-system, and not provable (it's totally meaningless to say "provable in the model"). At the same time, the completeness theorem tells you that every sentence that can neither be proven nor disproven must have two models disagreeing on (the truth value of) that very sentence. – user21820 Apr 16 '17 at 16:21
  • @AliquidExNihilo: By the way, intuition is misleading, and both your implicit claims about absolute truths and self-consistency-proving systems can be considered false. Here is an absolute truth that I guarantee you cannot deny: Something exists. Also, there are in fact self-verifying formal systems that are consistency and yet can prove their own consistency. You're going to have to get down into the details of the incompleteness theorems if you really want to know them properly; philosophy alone is simply inadequate. – user21820 Apr 16 '17 at 16:47
  • @user21820: Can you please illustrate distinction between semantic truth and syntactic proof? From the perspective of the meta-system, if statement is true and provable isn't it proved within (using model dependent rules/axioms) the model? I disagree with your claim "Something exists". I'm not sure if the physical universe(s) really exists. Just because there are humans (who may or may not really exist) in an universe (that may or may not really exist) who can think up things like logic doesn't guaranty something exists. Its just evidence towards "Something exists" but doesn't guaranty it – Aliquid Ex Nihilo Apr 17 '17 at 03:40
  • @user21820: Also in the wiki link of self-verifying formal systems it states "Provability of consistency can then simply be added as an axiom". That is a cop out, we can assume anything and say it's proved since it was assumed. – Aliquid Ex Nihilo Apr 17 '17 at 03:46
  • @AliquidExNihilo: Stop making hasty and faulty assumptions. I never said the physical universe exists; I just said "something exists". If you disagree, your disagreement exists. So you're wrong. And you don't understand the incompleteness theorems; you cannot add an axiom for self-consistency to any formal system that interprets arithmetic, like PA. So you're wrong, again. – user21820 Apr 17 '17 at 03:49
  • @AliquidExNihilo: As for semantic truth and syntactic proof, go and read the resources I recommended, as all this is very basic logic and it's a waste of time for me to explain to you what someone else already wrote up in detail. Try Stephen Simpson's notes first, and come to the logic chat-room if you have questions about it. If you wish to continue any discussion with me, I expect you to put in the effort to learn logic first. All the best. – user21820 Apr 17 '17 at 03:50
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    @user21820: There is no reason to get riled up, if you think I'm wasting your time then don't respond. I will stop asking you and responding to your comments from now on. BTW you are the one that is making hasty and faulty statements. I NEVER said that you said that physical universe exists, read my comment carefully again, I merely gave that as an example of evidence towards something exists. You have repeatedly misstated (or misinterpreted) my comments and other users like Rob Arthan and Noah Schweber have pointed that out in their comments. – Aliquid Ex Nihilo Apr 17 '17 at 04:01
  • @AliquidExNihilo: Ironic that you're the one judging my claims as hasty and faulty when yours are. You disagreed with my claim and gave the physical universe as "just evidence towards [the claim] but doesn't guaranty (sic) it". In my response I never said that you claimed that I said the physical universe exists. I just emphasized the difference, and that the reason it is guaranteed has nothing to do with the physical universe. The reason is rather that your very self (with your thoughts) constitute absolute evidence that something exists. – user21820 Apr 17 '17 at 04:08
  • @AliquidExNihilo: Also, you have the cheek to call Dan Willard's work a cop out when it is not. If you knew even a modicum of logic, I could make it clear for you, but I doubt you would understand what I'm going to say. For the sake of future readers, here it is. There is no formal system $S$ that interprets PA and proves $Con(S)$. However, there is in fact a formal system $T$ that interprets only part of PA and proves $Con(T)$. You can't just "add an axiom to cop out" as I stated already in my answer; $S'=S+Con(S)$ does not prove $Con(S')$. Just go and learn logic....... – user21820 Apr 17 '17 at 04:11
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    @user21820: You are the one that felt the need to justify that "I never said the physical universe exists". My comments didn't imply anything about your thoughts on physical existence - yet you felt the need to include that in your response and it was completely irrelevant like your answer and other comments. Evidence of something is not a guaranty (its not a typo, google it) of anything. My comment on self-verifying formal systems has nothing to do with PA yet you want to interject into the conversation. My question has nothing to do with incompleteness yet you keep bring that up. – Aliquid Ex Nihilo Apr 17 '17 at 05:07
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Firstly, you must realize that the incompleteness theorems have the general form:

For every formal system $S$ satisfying certain conditions, there exists a sentence that $S$ cannot prove.

They do not say something like :

There exists a sentence that every formal system satisfying certain conditions cannot prove.

So, you cannot go and change $S$ without also expecting the unprovable sentence given by the incompleteness theorem not to change!

In particular, taking any formal system $S$ satisfying the "certain conditions", it is trivial to let $S' = S + \text{Con}(S)$. Then $S'$ certainly proves $\text{Con}(S)$ even though $S$ does not, but by the same incompleteness theorem $S'$ does not prove $\text{Con}(S')$.

Since you say you do not know where to look, I recommend these resources, which you can go through in order and skip those that are too easy for you. (These are so far the best free online resources I have found.) However, there are alternative perspectives not found in any of these books, including resolving logic paradoxes and computability-based proof of the incompleteness theorems.

Note that there is no shortcut to full understanding of these theorems of logic. If you know both programming and basic first-order logic, you can jump straight to the computability-based proof to get a high-level grasp of it, before you get into the details of coding everything into arithmetic. But either way it will take time to work through the details.

user21820
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  • I think you have misunderstood the question. The OP is asking if you can find a consistent formal system that proves some given statement. – Rob Arthan Apr 10 '17 at 19:36
  • @RobArthan: Well I think I have understood the question, since I've seen enough such questions about the incompleteness theorems to know what the askers are after. A few examples of this kind here and here and here. Besides, the title of this question says it all: Can we change the formal system itself to prove its own consistency while maintaining its consistency? Which I have hence answered. =) – user21820 Apr 11 '17 at 04:19
  • The title of the question does not mention the system proving its own consistency. – Rob Arthan Apr 11 '17 at 05:19
  • @RobArthan: You don't get my point. People who do not understand the incompleteness theorem always think they can get around it. It is clear enough why the asker asks "can ... be proven by changing..."... – user21820 Apr 11 '17 at 05:24
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    OK. I'll leave the mind-reading to you. Read literally the title of the question has nothing to do with a system proving its own consistency. It's just saying given $\phi$, can I change some consistent system so it proves $\phi$. To which the answer is "not in general, unless you don't care about the intended meaning of $\phi$ in the original system". No lecture on the incompleteness theorem is required to see this. – Rob Arthan Apr 11 '17 at 05:29
  • @RobArthan: Yes I agree that as stated the titular question has nothing to do with the incompleteness theorem, and I'm doing mind-reading to guess the real question. =) – user21820 Apr 11 '17 at 05:42
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    @User21820: The question has nothing to do with proving self consistency. You are correct that question doesn't have much to do with Godel except that I thought about it while reading the proof. I understand you are piqued with newbies trying to skirt Godel's theorem (which I'm not). Even before I learned about Godel's theorem, I always wondered about what 'metrics' could be used to label something as 'true' (other than proving or assuming it). If something is true in the absolute sense then we are getting something from nothing. Sorry for the digression - just wanted to minimize mind reading. – Aliquid Ex Nihilo Apr 12 '17 at 08:30
  • @AliquidExNihilo: Okay. But you may be interested to know that the answer to your question is still the same. Namely, there is no such thing as absolute arithmetic truth. Ultimately, the meta-system MS must have some collection that is a model of PA. From its viewpoint MS believes that that model is the true collection of natural numbers, and measures truth against that. But MS itself could have other axioms that make its understanding of natural numbers wrong to a different meta-system. Worse still, it is entirely possible for MS to think that MS proves some wrong arithmetic sentence! – user21820 Apr 12 '17 at 14:46
  • Even if MS is consistent. There's no digression for you to be sorry about anyway. And if you're already familiar with basic logic and computability, you would certainly benefit from the computability perspective because it shows the same issue; in MS we define what is a program, but every program is a finite string, which can be coded and manipulated as a natural number. So MS believes that there is no program (based on its understanding of natural numbers) that can compute the truth value of every arithmetical sentence (which is also defined based on the same 'natural numbers'). – user21820 Apr 12 '17 at 14:51
  • @AliquidExNihilo: In other words, there is no possible absolute notion of natural numbers! =) – user21820 Apr 12 '17 at 14:52