# Defining the standard model of PA so that a space alien could understand

First, some context. In one of the comments to an answer to the recent question Why not adopt the constructibility axiom V=L? I was directed to some papers of Nik Weaver at this link, on conceptualism. Many of the ideas in those papers appeal to me, especially the idea (put in my own words, but hopefully accurate) that the power set of the natural numbers is a work in progress and not a completed infinity like $$\mathbb{N}$$.

In some of those papers the idea of a supertask is used to argue for the existence of the completed natural numbers. One could think of performing a supertask as building a machine that does an infinite computation in a finite amount of time and space, say by doing the $$n$$th step, and then building a machine of half the size that will work twice as fast to do the $$(n+1)$$th step and also recurse. (We will suppose that the concept of a supertask machine is not unreasonable, although I think this point can definitely be argued.)

The way I'm picturing such a machine is that it would be a $$\Sigma_1$$ oracle, able to answer certain questions about the natural numbers. I suppose we would also have machines that do "super-supertasks", and so forth, yielding higher order oracles.

To help motivate my question, suppose that beings from outer space came to earth and taught us how to build such machines. I suppose that some of us would start checking the validity of our work as it appears in the literature. Others would turn to the big questions: P vs. NP, RH, Goldbach, twin primes. With sufficient iterations of "super" we could even use the machines to start writing our proofs for us. Some would stop bothering.

Others would want to do quality control to check that the machines were working as intended. Suppose that the machine came back with: "Con(PA) is false." We would go to our space alien friends and say, "Something is wrong. The machines say that PA is not consistent." The aliens respond, "They are only saying that Con(PA) is false."

We start experimenting and discover that the machines also tell us that the shortest proof that "Con(PA) is false" is larger than BB(1000). It is larger than BB(BB(BB(1000))), and so forth. Thus, there would be no hope that we could ever verify by hand (or even realize in our own universe with atoms) a proof that $$0=1$$.

One possibility would be that the machines were not working as intended. Another possibility, that we could simply never rule out (but could perhaps verify to our satisfaction if we had access to many more atoms), is that these machines were giving evidence that PA is inconsistent. But a third, important possibility would be that they were doing supertasks on a nonstandard model of PA. We would then have the option of defining natural numbers as those things "counted" by these supertask machines. And indeed, suppose our alien friends did just that--their natural numbers were those expressed by the supertask machines. From our point of view, with the standard model in mind, we might say that there were these "extra" natural numbers that the machines had to pass through in order to finish their computations--something vaguely similar to those extra compact dimensions that many versions of string theory posit. But from the aliens' perspective, these extra numbers were not extra--they were just as actual to reality as the (very) small numbers we encounter in everyday life.

So, here (finally!) come my questions.

Question 1: How would we communicate to these aliens what we mean, precisely, by "the standard model"?

The one way I know to define the standard model is via second order quantification over subsets. But we know that the axiom of the power set leads to all sorts of different models for set theory. Does this fact affect the claim that the standard model is "unique"? More to the point:

Question 2: To assert the existence of a "standard model" we have to go well beyond assuming PA (and Con(PA)). Is that extra part really expressible?

• We don't need aliens to create doubts about communicability and sensibility of the notion of a standard model. May 22, 2019 at 22:16
• @RodrigoFreire $L_{omega_1,\omega}$ is indeed enough, and so are some other logics, like first-order logic augmented with the least-fixed-point operator. But I don't volunteer to explain to the aliens what those logics are and how to understand the formulas of those logics. May 23, 2019 at 0:30
• @RodrigoFreire To be more clear, I actually like your comment. If we need to assume the existence of an uncountable ordinal in order to talk about the "actual" natural numbers, so be it. I have to admit that this would be surprising to me. May 23, 2019 at 0:42
• @AndreasBlass And how do we understand "only finitely many" without access to the natural numbers? May 23, 2019 at 5:03
• @NoahSchweber If "finitely many free variables" were the only problem, we could get around it by saying that any formula must be a subformula of a sentence. But the real problem is "obtain" (along with "countable") because if the aliens understood "obtain" as we do, then we could define the natural numbers as the things obtainable from 0 by the operation of successor. That's one of the reasons I mentioned least-fixed-point logic in my earlier comment. May 23, 2019 at 11:04

These are fundamental questions. We know that any computable set of axioms which holds of the natural numbers must also have nonstandard models. But, paraphrasing Hilary Putnam, if axioms cannot capture the "intuitive notion of a natural number", what possibly could?

As I see it, there are two possible positions on this. One is that we do know what the natural numbers are, and the fact that axioms cannot capture them shows some limitation in the axiomatic method, not in the concept of number itself. The other is that our inability to capture the natural numbers axiomatically shows that we do not actually have a definite conception of them.

I hold the first view, but I admit that the second has its appeal. Asking how we could communicate the idea of a standard model to aliens brings home the difficulty of affirming that we have a clear conception of something while admitting that we are unable to communicate it through language (in particular, axioms).

This leads to deep philosophical questions about how we can communicate anything through language. Cf. Wittgenstein's "private language" argument and his ideas about rule-following.

Here are some things I would say in defense of the view that our conception of $$\mathbb{N}$$ really is definite, despite the fact that we cannot capture it with (first order) axioms:

1. Skepticism about the natural numbers can be ramped up. What do you say to someone who denies that we have a clear conception of $$10^{100}$$? There are serious people who would say we don't. Frankly, I think I have a clearer conception of $$\mathbb{N}$$ than of $$10^{100}$$.

2. My sense is that everyone accepts that our conception of the natural numbers is definite until they learn the incompleteness theorems, but some people are so impressed by these results that they abandon the idea that there even is a definite set of natural numbers. But Wittgenstein's rule-following paradox shows that even axioms may lack the definite character we ascribe to them. So why would we take them as the be-all end-all?

3. Taking the view that anything meaningful is captured by axioms, and thus that $$\mathbb{N}$$ is indefinite, has some unpleasant consequences. Say you take the view that there is no distinguished "standard" model of PA: all that matters is what statements can be proven from the Peano axioms. Then you have to accept that "what statements can be proven from the Peano axioms" is itself indefinite. Because the length of a valid proof in PA is a natural number, so if we don't know what the natural numbers are then we don't know what are the possible lengths of proofs. There could be "proofs in PA" which are valid on one version of $$\mathbb{N}$$ but not on another. Can you really swallow this?

4. I think the strongest argument in favor of the definiteness of $$\mathbb{N}$$, and against the idea that PA, or any other axiomatization, is the be-all end-all, is the evident fact that we have an open-ended ability to go beyond any computable set of axioms, for instance by affirming their consistency. If you accept PA you should accept Con(PA), and the process doesn't stop there: you can then accept Con(PA + Con(PA)), and so on. This goes on to transfinite levels. If our understanding of $$\mathbb{N}$$ really were fully captured by some particular set of axioms then we would not feel we had a right to strengthen those axioms in any special way; the fact that we do feel we have this right shows that our understanding is not captured by any particular set of axioms.

This is my view.

• I appreciate these ideas. For beings with access to an oracle of a model where Con(PA) is false, they would argue (presumably) that our conception of consistency is not reflected in reality. So to them, #4 might be question-begging. To them, #3 is moot because $\mathbb{N}$ is not captured by the axioms, but is captured by their machines. But even assuming their machines did say that Con(PA) is true (and Con(PA+Con(PA)), and so on as far as we tested) could we be sure that the supertask machines are really getting the natural numbers? I'd argue no. May 23, 2019 at 16:34
• Point 4 is obvious in retrospect, given the second incompleteness theorem, but I hadn't quite looked at it that way. And it applies not just to PA but to any reasonably powerful theory T. Thus it seems to really drive home the limitations of the axiomatic method. May 26, 2019 at 4:32
• @AndreasBlass: what makes me uncomfortable is the idea that one can "intuitively" understand something but not be able to express that understanding through language. Maybe there's no problem here, but I'm not sure there's no problem. May 27, 2019 at 18:46
• @NikWeaver, I think the aliens from the first-order land of $PA+\neg Con(PA)$ say to us "you've agreed to accept arbitrarily long proofs, so you must accept this proof which you call non-standardly long; you have not articulated any criterion of length by which to reject it." In any case, I like this version of the question; "do you accept a non-standardly long proof of $0=1$?" feels much sharper than "do you accept non-standard numbers?"
– user44143
May 29, 2019 at 11:27
• "Then you have to accept that 'what statements can be proven from the Peano axioms' is itself indefinite. Because the length of a valid proof in PA is a natural number, so if we don't know what the natural numbers are then we don't know what are the possible lengths of proofs. There could be 'proofs in PA' which are valid on one version of N but not on another." This seems incorrect. The length of an actual proof in PA is not an object-language natural number. It is a metalanguage natural number. These can be thought of as corresponding to a sorites-like subcollection of $\mathbb N$. Nov 16 at 10:12

Here in the realm of mathematical fiction....

I imagine aliens who do not have the natural number that we call 13. They also lack 41 and 1681 and many other numbers. They do not have the successor or addition functions.

Instead...they regard multiplication and comparison as fundamental. They don't know how to add possibilities, but they know how to multiply and compare them. They rarely count, but they have a keen eye for when one rectangle is bigger than another. So they often arrange things into rectangles, and they regard it as the professional duty of bakers to sell goods in 3x4 boxes.

Instead of Peano arithmetic, these aliens have a first-order theory in the language $$(1,\cdot,<)$$. Their $$<$$ has the same properties as ours; their $$\cdot$$ has the same universal properties as ours; and their $$<$$ and $$\cdot$$ are compatible.

They believe in the number $$2$$, i.e. the unique number $$x$$ such that $$1 Similarly they believe in the numbers $$3$$ and $$7$$, and $$12=2\cdot 2\cdot 3$$ and $$14=2\cdot 7$$. But they can detect a gap between $$12$$ and $$14$$ because $$\exists k\, \exists x\, (12 x < k x < 14 x) \wedge \nexists k\, (12 < k < 14)$$

Their induction takes the form $$P(1) \wedge \forall x(\forall y(y

They may define $$x+y=z$$ by something like our formula $$S(xz)S(yz)=S(zzS(xy))$$ but this does not play a large role in their arithmetic.

Finally, we observe an embedding of their natural numbers into ours, uniquely fixed by the desire to preserve both $$\cdot$$ and $$<$$. But if they believe that there is no number in between $$12$$ and $$14$$, and they have lived happily without any such need, what are we to say?

Meanwhile here in New York...I learned how to count from the numbers on an elevator in the apartment building, and there was no 13th floor.

• "But if they believe that there is no number in between 12 and 14, and they have lived happily without any such need, what are we to say?" Nothing. Their number system is just fine for their purposes, and they can grok our notion of 13. The situation is different, I believe, when someone's notion of "finite" comes from access to an oracle (to a nonstandard model). How do we explain what a "standard" finite number is? That is, how do we define "finite" without an a priori understanding of it? May 23, 2019 at 14:33

The answer to this question depends on how skeptical you want to be.

Let's consider a simpler task. Can we communicate to the aliens what we mean by "3"? It's possible to imagine a scenario in which we are unable to do this. Imagine for example that the aliens don't look like human beings and don't communicate using anything like a human language, but instead resemble ants. We can imagine trying to train the aliens to understand "3" if we are able to identify something that they desire (food, for example) and putting food inside boxes marked with 3 copies of the same symbol, while leaving other boxes empty. After a sufficiently long training period, we might become convinced that the alien ants have mastered what we mean by "3", but how could we ever be sure? It's quite conceivable that the next time we put 3 symbols on a box, they might get confused. Perhaps we use a new type of symbol that they have trouble parsing. Or perhaps we use exactly the same type of symbol as in the past, but it turns out that the concept they have learned from us is, when translated into our language, something like "3 provided there is currently no syzygy of three planets, and 4 if there is currently a syzygy of three planets" (cf. Goodman's grue paradox). In principle we might detect such a thing if our training period were sufficiently long, but we would still be unsure whether some other clause is "hiding" in there, manifesting itself even more rarely than planetary syzygies do.

If "3" cannot be communicated, then certainly something infinite like the natural numbers cannot be communicated. But suppose we grant that somehow, "3" can be communicated. Could we communicate the standard model of PA as opposed to some nonstandard model of PA?

My view is that it is a very peculiar kind of skepticism that regards nonstandard models of PA as posing a particular problem for communication tasks. A nonstandard model of PA is, by any conceivable measure, a more complicated concept than the standard model of PA. For starters, what do we even mean by PA? PA contains an induction schema. Communicating what we mean by the induction schema for PA is no easier than communicating what the standard model is. If you think there is a difficulty disambiguating between PA and nonstandard models of PA, then how do you explain to someone that when you say "PA" you mean that the induction schema ranges over formulas whose length is a standard natural number as opposed to formulas that are nonstandardly long?

To be sure, you are free to be skeptical that we are able to communicate what we mean by PA. But this kind of communication difficulty is more fundamental than the difficulty of communicating the difference between a standard and a nonstandard model of PA, since the latter must at least presuppose that we know what PA is.

Another way to see that PA is a red herring is to replace PA with Robinson's arithmetic Q. Can we communicate to the aliens the distinction between a standard and a nonstandard model of Q? The same difficulty with infinity arises; specifics about PA are irrelevant.

It comes down to a question of whether we think we can communicate something infinite, and if so, why it is that we think we can do so. Technical details about logic and arithmetic are a distraction.

• Good points. This reminds me of that old Chinese room problem. Anyway, if it helps get rid of distractions, here is another formulation: You have (presumably) a picture in your head of the natural numbers, as do I. We try to express these pictures using finite symbols, sounds, words, etc... to each other. We seem to agree that we have the same picture. Are we justified in claiming that we in fact do have the same picture? If not, is anyone really justified in using the phrase "standard model of the natural numbers"? If so, what justifies it? May 23, 2019 at 16:56
• @PaceNielsen : Yes, that is a better way of phrasing the question. I would say that whatever it is that we appeal to for justification, it cannot be mathematical proof in the usual sense, because the claim that your internal mental picture matches my internal picture (in some sense at least) is not a mathematical statement. This observation may make some uncomfortable, because mathematicians like to think of themselves as relying solely on "proof" and not on any murky philosophical assumptions. But we are deluding ourselves if we think that mathematics is free of philosophical assumptions. May 23, 2019 at 17:51
• @PaceNielsen (cont'd): What we can say is that mathematicians who have been suitably trained in the linguistic usage of the term "standard model of the natural numbers" are able to reach agreement about which mathematical inferences employing that phrase are valid/invalid, in exactly the same way that they are able to reach agreement about inferences in any other area of mathematics. The empirical fact that the mathematical community is able to reach a high level of agreement about which inferences are valid is often taken to be a justification of the meaningfulness of mathematical activity. May 23, 2019 at 22:16
• @PaceNielsen Before asserting or denying that my mental picture of $\mathbb N$ is the same as yours, I'd want a clear notion of what it means for pictures in two minds (or brains) to be the same. That notion might have something to do with homologous neurons firing in similar circumstances, but I think neuroscience today (and not merely my knowledge of it) is far from making such a notion precise. In other words, I don't even think it makes sense to ask whether two people have the same picture of $\mathbb N$ (or of anything else). May 27, 2019 at 17:53
• I suspect that aliens if they have no prior notion of the number 3, there would be no reason for them to notice whether there are exactly three symbols or not to mark a box with a desirable content. Especially if they automatically perceive a combination of three different symbols as an entirety, or, conversely, if they perceive each symbol to be composed of a variety of subelements. Sep 19, 2019 at 17:39

Re: question 1, there is a precise sense in which this task is impossible, but we can arguably get a "dynamic approximation."

First the negative response. Since first-order logic isn't enough, a case can be made (which I would agree with incidentally) that there is no satisfactory way to communicate this. A relevant result for this general principle is Lindstrom's theorem, which intuitively says that any logic stronger than first-order logic must be fundamentally "infinitary," by virtue of not having a finitary proof system or by virtue of having to take into account uncountable structures even at the most basic level.

Now the positive response. Although prima facie it involves an appeal to a large fragment of the set-theoretic universe, we can think of the "from above" characterization (= the smallest model of PA) as a dynamic communication. The idea being that whenever you and I have possibly non-isomorphic models of PA, we each try to find embeddings of our own model into the other. If one of us succeeds and the other fails, then we tentatively agree that that person is holding the standard model. If both of us succeed, then we tentatively agree both are. And if we both fail, we tentatively agree that neither of us is holding on to the standard model. Note that while this process can be wildly fluctuating, it is stable in the following weak sense: if you are holding the standard model and I'm holding a nonstandard model, we won't possibly believe that my model is standard.

Of course, there is an appeal to quantification here. Namely, checking whether a purported embedding is actually an embedding involves a universal quantifier. But in this case we're only quantifying over things we already have; in particular, we can make perfect sense of this even if both of us are holding nonstandard models.

So I think this isn't entirely silly, although I do believe it's lightyears away from satisfying (and indeed that the actual answer is that no satisfying method exists).

Re: question 2, what do you mean by "expressible?"

We can easily write a formula in the language of set theory asserting "$$x$$ is the smallest inductive set," and so in that sense we have expressibility in the language of set theory.

On the other hand this definition is very model-dependent, with nonisomorphic models of set theory giving potentially non-isomorphic "smallest inductive sets." By the compactness theorem this is unavoidable as long as we stay in the realm of first-order logic, so that can be interpreted as a negative answer.

• By "expressible" I meant definable in a non-circular way. Your comment above, about understanding "only finitely many" only after having access to the natural numbers gets to the heart of it. May 23, 2019 at 14:14
• It seems to me to be pretty clear you can't define the natural numbers, in any reasonable sense, or in any sense agreeable to you, in a non-circular way. So - next question? I hope I'm not seeming to be flippant here, but still.
– abo
May 23, 2019 at 15:24
• I think we can be absolutist about the notion “smallest inductive set.” It’s true that this notion can be interpreted differently in different models of set theory, but the concept of algebraic generation is so primitive and clear. Further, the standard numbers always embed into any nonstandard version, so there is some absolute, inter-universal sense in which it is “smallest.” May 23, 2019 at 16:57
• @PaceNielsen. I guess they talk about it because in a context such as ZF, or some such theory, it has a meaning (the one Monroe is talking about). That is, for mathematicians working in the context of ZF, it makes sense to talk about the standard model, and it helps them to do mathematics.This doesn't provide a "definition" in your sense, however.
– abo
May 23, 2019 at 17:58
• @PaceNielsen : If you're complaining about the choice of the word "standard," I don't think it's any different from the way the adjective "normal" is overused in mathematics. What's "normal" about the "normal distribution"? Or is your complaint that if something can't be defined non-circularly then we can't know what it is? But in that case, I'd claim that the number "3" can't be defined non-circularly either, yet I don't believe we can't know what "3" is. May 23, 2019 at 20:17

Just an expanded comment. The standard model of PA can be characterized as the unique (up to isomorphism) model of PA such that all of its individuals are suitably named by numerals (like SSS...S0). Therefore, we can understand the standardness of the standard model as the matching between its individuals and the “metamathematical numbers.” Now, what if the metamathematics of aliens is nonstandard? As I see it, the metamathematics is not a model, so it is neither standard nor nonstandard. The metamathematical numbers are just subjects of the metamathematical discourse. Therefore, if we can explain the matching between the “metamathematical numbers” and the individuals of the standard model, then we can explain the standard model, because this matching can be seen as the essence of its standardness.

• But "SSS...S0" is not actually a numeral. To define "numeral" don't you already need access to the natural numbers, thus making the definition of the standard model circular? May 23, 2019 at 14:28
• I am using SS...S0 as a numeral, a term of PA determined by a metamathematical number (the metamathematical number of S’s). May 23, 2019 at 14:32
• Are metamathematical numbers somehow prior to actual numbers? Seems circular to me. May 23, 2019 at 14:36
• Precisely. What are the actual numbers? You apparently need them to talk about formal systems, and metamathematical numbers. I might say "things that count". The aliens might say "the possible steps taken by a supertask machine" (or in other words, the numbers $n$ such that a supertask machine says that $\exists n...$ gives a positive answer). The aliens might ask us how our notion differs from theirs. Could we express how they differ? (Do they differ?) May 23, 2019 at 14:52
• Yes, that is about it. The metamathematical numbers are not a model, therefore neither standard nor nonstandard. Models are mathematical objects living inside a mathematical theory (of sets) and modeling formal systems like PA. The metamathematics is not like that. May 23, 2019 at 17:19

So the aliens taught you how to build a device and interpret its outputs as statements about first-order number theory. They say that it is performing "supertask" computations, or at least that's how we've translated their claims into Scientific English. But since we cannot check for ourselves, with our dull senses and limited memories, how do we know that's what's happening?

This is a serious question, because the assumption of the OP is that the device is somehow checking against some model of PA. Why do we believe the outputs correspond to what is satisfied in an actual model of PA? And which model? My point is that the particular reasons we have for believing in the reliability of this infinitary computer, or our particular understanding of how it is working, would be key to shedding light on whether the outputs are about a standard or nonstandard model of PA.

It could also give us the vocabulary to explain to the aliens what we mean by "standard model." Because if they have any knowledge of models of PA, enough to actually "touch" one, then surely they know that are many different ones. So we should be able to ask them why the computer tracks truths about their preferred model of PA and not a different one. This will have to depend on how the particulars of this sci-fi story are elaborated.

• "But since we cannot check for ourselves, with our dull senses and limited memories, how do we know that's what's happening?" Clearly, we don't. But we can see the idea of it (a machine building a smaller machine, etc...). So that gives us a mental picture. But the point is, as you have grasped, that our mental pictures of the infinite might not match what reality would do with an infinite process. May 23, 2019 at 14:25
• @PaceNielsen So it's doing some unknown thing, physically somehow. Why even believe this has to do with PA? It looks like it's computing some numbers, going faster and faster, and then, FLASH BANG, it sings a song about RH. Maybe the aliens just have a nice device for dazzling math fans. What do we even mean by "what reality would do with an infinite process?" I mean, what is it even doing? Does it have anything to do with number theory in the end? Or is it just a fireworks show? There's not enough information in the story. May 24, 2019 at 8:07

If PA is inconsistent, then by Gentzen's consistency proof, this means that the logical system of primitive recursive arithmetic (PRA) augmented with the well-ordering of ϵ₀ is inconsistent. The consistency of PRA is, I'm pretty sure, beyond reasonable debate, so the more likely explanation is that ϵ₀ is not well-ordered, and thus has an infinite descending sequence beneath it. Given that every ordinal below ϵ₀ can be expressed as a finite rooted tree, there would have to be some "finite" rooted tree in the aliens' system which has a nonstandard number of nodes, and thus can be decremented from forever.

I think that the existence and well-orderedness of ϵ₀ would be something they could learn about and accept, and any model that doesn't prove it is not the standard model.

I think the answer to your question really depends on how we take ourselves to understand a canonical model of PA. Ultimately I think it comes down to the way language about mathematics is grounded in a kind of modal semantics for physical procedures.

Consider a human child learning arithmetic for the first time. The first thing they might learn in a contemporary mathematics curriculum is how to associate physical arrangements of marks/dots with arbitrary arrangements of distinguishable objects. A basket containing six oranges can be associated with six equally spaced dots on a line, like $$*$$ $$*$$ $$*$$, or six equally spaced dots in a rectangle, like:

$$\begin{matrix} * & * \\ * & * \\ * & * \end{matrix} \text{ or } \begin{matrix} * & * & * \\ * & * & * \end{matrix}$$

They learn that adding a single item to the basket corresponds to adding an additional dot on the end of a line of dots, and removing an single item from the basket corresponds to removing a dot from the end of a line of dots. They learn how to rearrange rectangles of dots into lines of dots. They then learn how to associate lines of dots with, say, arrangements of decimal numerals and verbal utterances. They then learn to associate addition with concatenation of lines of dots, and multiplication with creating a rectangle of dots whose sides are associated with the multiplicands. They learn a procedure for manipulating two arrangements of decimal numerals to to produce an arrangement of decimal numerals representing their sum or product (the usual addition and multiplication algorithms), and verify by many examples that these physical procedures on arrangements of numerals correspond to the previous physical procedures on arrangements of dots.

What are they actually learning here, though? It looks like they are learning certain modal claims about physical procedures, essentially by induction in the sense of Hume. They learn, for example, that given two equally spaced lines of dots, it is always possible to form their concatenation. They learn that the procedure for association "a+b" to an arrangement of dots and "b+a" to an arrangement of dots necessarily result in arrangements of dots which, if they were to verbally count them, end at the same verbalization. I would claim they are learning these modal claims in the same way a physicist might learn certain modal claims about the motions of physical objects, like "For any possible closed physical system, momentum is conserved". I think these are rightly considered modal claims, not just universal claims, because they are about what would happen, not what is: we are learning about what would happen if we were to do various physical procedures, not about what is (of course this is an anti-Platonist view).

So how does this relate to your original question about aliens? Well, numbers correspond to arrangements of equally spaced dots that we could create (in a somewhat liberal interpretation of "could"). As mathematicians, we often assume that there is no limit to the amount of time or resources we have to perform a manipulation of symbols (which is expressed by the totality of the successor function -- for any linear arrangement of equally spaced dots, we always could add another dot to the end). We believe that this modal language is unambiguous, because it seems to connect directly to our experience of the physical world we live in.

But there is a good argument this is not so, or at least the naive interpretation of these modal claims as physical possibility does not work: there is a finite amount of matter and energy in the universe with which to create these arrangements of dots.

However, there still may be something about our universe which allows these modal claims to be grounded in something "real". If our universe was Newtonian or Minkowskian, then in a physical line segment of unit length, there exist arbitrarily large finite collections of non-zero points along that segment whose spacing between adjacent triples pairs of points $$a < b < c$$ in that collection has the distance between $$b$$ and $$c$$ equal to half the distance between $$a$$ and $$b$$. And any infinite such collection has the same order type. So we can find the order-type of the natural numbers, essentially, in the level of saturation of a unit interval of space (not quite in the model theoretic sense, but related). We might also believe time is unbounded, and that for any full time-like geodesic in our Minkowskian universe, any unbounded collection of equally-spaced points in time with an initial point is order-isomorphic to the natural numbers. On a theory of meaning based on the right notion of reference, our language could thus pick out a canonical model of arithmetic by reference to our actual universe.

Not sure about aliens but if we had to approach the task of explaining $$\mathbb N$$ to, say, the Pirahãs (who have presumably not been exposed to any modern mathematics), we would first have to answer Hamkins' question to the "mathematical oracle":

Do we have an absolute notion of "finite"?

Since the answer seems to be likely negative, the corresponding answer to your question is that the task would be impossible. Of course, the Pirahãs, who have the same cognitive capacity at birth as all humans, can be replaced here by anybody who has not been exposed to modern set theory, e.g., Leibniz, who would have rejected the formula $$\mathbb N\in \mathcal P (\mathbb N)$$ as meaningless even after getting all the necessary explanations with regard to the notation, because he viewed "infinite wholes" as contradictory entities (namely, contradicting the part-whole principle, along the lines of Galileo's paradox with $$n\mapsto n^2$$). On the other hand, Leibniz would have agreed that infinitesimals are more complicated than 1,2,3,... Indeed, the counting numbers are assignable, whereas infinitesimals are inassignable, in Leibniz's terminology.

• Irrespective of any deep philosophical musings, in practice we clearly communicate the idea of numbers to other humans all the time. It seems bizarre to think there is a group of humans who we couldn't, with some effort, communicate the idea of numbers to. Nov 15 at 13:26
• @SamHopkins: While I see why you think it seems bizarre, here is a more advanced analogue that may explain why it may not be bizarre: Mathematicians of a certain era thought that infinitesimals are problematic and they avoided using them. Then came Robinson... One may say that Robinson would have hard time communicating the idea of consistently doing calculus with infinitesimals to, say, Bishop Berkeley. The only difference here is that $\mathbb{N}$ seems more "basic" but that's just a habit of thinking. Is it really? Nov 16 at 10:06
• @SamHopkins: I think anecdotal and anthropological evidence is against you. "Understanding what strings are" is prior to counting: a child who asks her father "what does two mean?" has a grasp of grammar but not of arithmetic. The Pirahã people apparently speak a complex language but are wilfully against abstractions such as numbers. Mikhail's post seems to be to be very pertinent. Nov 16 at 20:47
• @SamHopkins: Counting numbers is easy, a child can do that. But understanding the smallest collection of objects that contains $0$ and $S$ is difficult, even for professional mathematicians, as shown by numerous discussions on MO. So my comment still stands that the set of natural numbers is no easier to understand than imagining points on a line that are infinitely close to $0$. Nov 17 at 11:32
• @Burak, in fact Leibniz considered such a "set" to be a contradictory notion (as explained in my answer), whereas infinitesimals are merely fictional (not contradictory). Nov 17 at 11:37

I'm going to have to tell you that describing your "standard model" is possible if and only if you believe that there is a true embedding of the naturals into the real world.

Specifically, if you do then you can simply define your "standard model" as precisely that embedding. For example, if you believe that one can build a true universal Turing machine that has an indefinitely extensible tape (not infinite but always can be extended) and that can run for an indefinitely long time without making a single error, then you can obviously define natural numbers as all finite unary strings that can be written by some instance of that Turing machine, and define the arithmetic operations on those unary encodings as the Turing machines that you can explicitly write down for them. There can then be no ambiguity at all what you mean by your "standard model".

However, if you hold a more 'physical realist' view, the finiteness of the observable universe strongly suggests that there is no embedding of naturals into the real world, and hence no Turing machine of the kind described above (for reasons of both space constraints and occasional infidelity). If so, then describing your "standard model" is impossible simply because it does not even exist! Think about it; you cannot even describe it uniquely to yourself, since no computable formal system can pin it down.

• To question whether one can "build a true Turing Machine" (in the "real world") does not make any real sense. A TM is an abstraction of how computation works. It is like asking whether one can "build the true number 0", or "build the true imaginary number i". Of course you can build a TM from examining the way it is defined as a tuple of sets in ZF and suggesting the appropriate sets to combine. Sep 22, 2019 at 14:42
• @TommyR.Jensen: Your comment does not make any sense. The fact that you can read this webpage (via HTTPS) rests on the correctness of Fermat's little theorem F when interpreted in the real world as proving a truth about some calculations in your computer involving 1024-bit integers or larger. This same interpretation can be used to interpret all theorems of PA (not just F). If such an interpretation actually works, then that is precisely what "true embedding of the naturals into the real world" means. Same for "true universal TM", which here clearly means a real-world embedding of a UTM. May 1, 2021 at 3:40
• By "true universal TM" I did not mean "true" in the sense of "one true" but rather "not fake". Your computer is not a true UTM because of finite memory; you need it to have an indefinitely extensible memory for it to be a "true UTM". May 1, 2021 at 3:41
• Your answer shows why describing a "standard model" is possible if there is a true embedding of the naturals into the real world. (I think "you believe" does not belong, because the way you propose to use that true embedding is to exhibit it physically, which you cannot do if you merely believe in it.) But why only if? Nov 14 at 14:38