# Is the axiom of infinity truly an axiom?

I hope I can communicate my concerns effectively, so I can reach an understanding about a topic that I've been reflecting and researching intensely on for a few days. I am thinking about actually infinity in mathematics, specifically set theory with respect to the axiom of infinity, and the set of natural numbers as a completed infinity.

I understand that there will not be answers to this post that will solve the question as to whether or not an actual infinity exists theoretically, but I hope to have answers that will spark insight in my mind.

Questions::

1. Is the axiom of infinity truly an axiom?

What I mean is that axioms are usually taken as self evident truths that need no proof, but I don't see how the axiom that an actual infinite set exists is truly self evident. It appears that it was created solely for allowing the possibility of infinite sets regardless of how self evident it may or may not be, which seems inconsistent with how axioms are meant to be used.

1. Are there plausible reasons to believe in the theoretical existence of a completed infinity?

I believe an actual infinite set does theoretically exist, like the set of natural numbers. I see nothing intrinsically wrong or ambiguous with the definition of the set of natural numbers, or with the inductive set used in the axiom of the infinite set. Further, I also believe that if a set is defined, all the elements satisfying the definition or property of that set already exist in that set even though we may not be able to enumerate them all even given infinite amount of time. Consequently, all the natural numbers are in the set containing them, which means this set is a completed infinity. Somehow I find it plausible to have a collection of infinitely many objects within a collection, but I am curious if anyone has found reasonable arguments to bolster his or her confidence that accepting this axiom is not just a leap of faith, but a rational thing to do.

I do hope that this question is philosophical enough in nature, or at the very least have the potential to spark philosophical discussion.

Thank you all in advance for any feedback.

• That the axiom of infinity is not self-evident was one of the criticisms of Russell, who tried to derive all of mathematics from "laws of thought" in Principia. However, the idea that axioms are supposed to be self-evident has been abandoned long ago, they are rather expected to be fruitful and useful in organizing the body of theory under consideration. Similarly, "theoretical existence" of completed infinity is not a question of belief but of practicality. Given how mathematics was practiced it was rational to reject it before Cantor, as it is rational to accept it now. Jun 9, 2017 at 4:54
• You'd probably be interested in Defending the Axioms by Penelope Maddy. Here is a relation question on math.SE. Here is Russell talking about the axiom of infinity. Jun 9, 2017 at 4:54
• "axioms are usually taken as self evident truths that need no proof". NO: they are assumed as "starting points" that we agree on without proof. Jun 9, 2017 at 5:57
• A "huge" part of math deals with infinity; thus, we need some assumption regarding the existence of an "initial" infinite collection. Jun 9, 2017 at 5:59
• The strongest rational support to the inexistence of a finite "amount" of numbers is in the very very fundamental intuition about the unlimited possibility of iterating the basic operation of +1. Consider the very simple game of asking to a boy: "Please, think at the biggest number you can imagine... Done ? Now add one to it." But you can consider Ultrafinitism: it seems that there is nothing intrinsecally "irrational" or inconsistent in it. Jun 9, 2017 at 6:03

Is the axiom of infinity truly an axiom?

Yes, it is an axiom of set theory.

But in mathematics an axiom of a theory does not have to be plausible according to our everyday intuition. The only requirement it has to satisfy: The axiom does not contradict the other axioms of the theory.

Of course axioms should not to be chosen arbitrarily. They should serve as the basis of a strong theory. Cantor's axioms on the existence of transfinite sets allow to extrapolate addition and multiplication to infinite sets and to distinguish between infinite sets with different cardinalities.

In my opinion, requiring plausibility of an axiom is a relict from a kind of philosophy which confines itself to the boundaries of our everyday intuition.

Are there plausible reasons to believe in the theoretical existence of a completed infinity?

What do you mean with the term "theoretical existence"?

On one hand, a mathematical object "exists" as soon as we have introduced it - i.e. invented it. It never exist in the physical world. On the other hand, a mathematical object can be a useful tool to describe phenomena in the physical world. But even then, the mathematical object is a model, it is not part of the physical world.

As stated in response to your first question I consider infinite sets a useful - even ingenious - invention within the domain of mathematics. In addition, each physical theory, which relies on calculus, argues with the set of real and complex numbers.

• A very good answer. And what I meant by theoretical existence is as you described the existence of a mathematical object; it exists, because we defined it. It is theoretical, because it doesn't exist in the physical world, but in the realm of pure reason and ideas. Jun 10, 2017 at 21:58

The other answer has covered the formal aspects. I will argue that with the right mental model, the axiom of infinity is 'self-evident'.

(I use scare quotes, because I believe the phrase 'self-evident' is merely an intensifier rather than something meaningful)

Set theory, as applied to foundations, is not about 'collecting' objects together — it is about doing logic. This manifests most strongly by looking at the axioms of extensions and comprehension, together with the construction of the third bullet

• S and T are the same set if and only if x∈S holds precisely when x∈T holds
• If Φ is any proposition, there is a set SΦ with the property that x satisfies Φ if and only if x∈SΦ
• If S is a set, then x∈S is a proposition that we can ask if x satisfies

Thus, the notions of set and proposition are just different ways of talking about the same thing.

(aside: this correspondence is somewhat spoiled by the fact unrestricted comprehension leads to paradoxes, but even that parallels the problems that logic has with the liar's paradox. But both set theory and logic have developed to deal with these glitches)

Thus, the very fact one finds "x is a natural number" to be a meaningful proposition one can ask of an object makes it evident that there is a corresponding set — and we call that set the set of natural numbers.

You may be interested in looking at type theory as a variation on the theme that tends to be developed more along the lines of formal logic.

• I am not quite convinced that set theory should be considered a sub-discipline of logic. Take typical concepts from set theory, e.g., the power set or the function: Do these concepts have pure logical terms as their counterpart? Jun 10, 2017 at 22:58
• @JoWehler: Yes; in the modern formulation that's basically the meaning of "higher order" in "higher order logic". E.g. in second order logic we can consider relation variables, and the predicate in R given by "∀x: if x satisfies R, then x is a natural number" is the predicate corresponding to the power set of the natural numbers.
– user6559
Jun 10, 2017 at 23:45
• How do you express the power set of N, i.e. the set of all subsets, by pure logical terms? Jun 11, 2017 at 0:21
• @JoWehler: Precisely by the predicate I gave in my previous post. In the set-predicate dictionary, "S is a subset of T" means "∀x: x∈S implies x∈T". Translating via the set-predicate dictionary gives "∀x : P(x) implies Q(x)". If you fix Q (e.g. to be "Q(x) := x is a natural number") that formula is a predicate in the variable P. Those P satisfying this predicate are precisely those predicates that correspond to the subsets of N.
– user6559
Jun 11, 2017 at 0:43

Formally, the axiom of infinity in standard set theory (ZFC) is as follows:

There is a set I such that it contains the empty set, and that whenever it contains a set x, it also contains the set {x}.

This is asserting the existence of a potentially infinite set, and not a completed one; it is saying for example, there is the set:

0, 1, 2, 3 ...

and not

0, 1, 2, 3 ... omega

Where omega is the completion of the foregoing.

That this is so, is shown by the direct adoption of this axiom in both intuitionistic and constructive set theory where the notion of potential infinity is more strictly adhered to.

Even in the standard ZFC there is a certain unease about the use of completed infinities, and this is often signalled by high-lighting the axiom of choice in text-books where this notion is most prominent; the axiom states I can make a completed infinity of choices, and not just a finite one; this was not felt to be as 'self-evident' as the other axioms.

Actually, this axiom in an indirect way was the cause of one of Feynmans 'jokes' on his mathematical friends where he pointed out to them that one of their theorems they were getting excited about (the Banach-Tarski Paradox) couldn't be true, he traced it to their use of infinite divisibility, saying this was physically impossible; in fact, he was stricter, saying that you can make a large number of cuts but not an arbitrarily high one, never mind a completed infinity of cuts.

Mathematically speaking, it's a form of ultra-finitism, as for example written about by the Russian mathematician Esenin-Volpin.

• I see your point. But is whether or not the set I in the axiom is a completed or potential infinity one of interpretation of the axiom? I have read some authors write in some set theory and real analysis books after having stated the axiom of infinity, that the axiom says at least one infinite set exists, namely the set N of natural numbers, and I don't think most mathematicians would think that N is not a completed infinity. So I guess my question is that if mathematicians assume the existence of a completed infinite set would they say that the axiom of infinity encapsulates this assumption? Jun 11, 2017 at 0:22
• Another question: Even if N is a potentially infinite set whose existence depends on the axiom, would mathematicians have to prove that in fact that N is an actual infinite set? I don't think it's possible for them to do that, unless they already presume that such sets exist, which would demand that they have an axiom like the one of an infinite set. Thank you for your patience with me inquiries. Jun 11, 2017 at 0:26
• Well, like I pointed out there is a distinction between 1,2,3... And 1,2,3, ... omega; and it's the existence of the first that is asserted not the second; mathematics text-books aren't known for being philosophical, they're there to teach technique and that is what they're for on the university curriculum; I suppose they take the practical view that before learning to question something, one should learn something; I remember being mystified why they kept referring to the axiom of choice as being controversial without explaining why. Jun 11, 2017 at 2:14
• There is after all a lot of technique to teach! For sure interpretations are important, but I'd point to the fact that this axiom is the same in the standard set theory and the intuitionistic as validating what I've written. The important thing to notice is that they don't tend to distinguish between the potential and actual infinite, the set N of natural numbers is a name for a potentially infinite sequence. Jun 11, 2017 at 2:22
• Sure, I would expand what you said a little and say that they assume by fiat the existence of an actual, potentially infinite set; from the other axioms they could not prove any such thing. Jun 11, 2017 at 2:32

But Where does it exist? I suggest that, like for instance identity, cause & effect, and perfect circles, it exists only in our minds and conceptual structures. We have a deep intuition that our 'self' is real, and use the concept as such, in a way that categorises it as real. Similarly infinity. But that is all real is, a conceptual category.