I studied math for a long time.
I’m convinced math would be better without infinity. It doesn’t exist. I also think we don’t need numbers too big . But we can leave those
However, I also am starting to believe that infinity doesn't exist.
Or more specifically, I want to argue that infinity is not a number, it is a process. When you say {1, 2, 3, ... } the "..." represents a process of extending the set without a halting condition.
There is no infinity at the end of a number line. There is a process that says how to extend that number line ever further.
There is no infinity'th prime number. There is a process by which you can show that a bigger primer number must always exist.
Sure, but ordinal numbers exist and are useful. It's impossible to prove Goodstein's theorem without them.
https://en.wikipedia.org/wiki/Ordinal_number
https://en.wikipedia.org/wiki/Goodstein%27s_theorem
The statement and proof of the theorem are quite accessible and eye-opening. I think the number line with ordinals is way cooler than the one without them.
I went down the rabbithole, and as far as I can tell, you have to axiomatically assume infinities are real in order to prove Goodstein’s theorem.
I challenge the existence of ordinal numbers in the first place. I’m calling into question the axioms that conjure up these ordinal numbers out of (what I consider sketchy) logic.
But it was a really fun rabbithole to get into, and I do appreciate the elegance of the Goodstein’s theorem proof. It was a little mind bending.
For example, S(0) is 1, S(S(0)) is 2, S(S(S(0))) is 3, and so on.
There is no end of a number line. There are lines, and line segments. Only line segments are finite.
> There is no infinity'th prime number. There is a process by which you can show that a bigger primer number must always exist.
You misunderstand the concept of infinity. Cantor's diagonal argument proves that such a bigger number must always exist. "Infinity'th" is not a place in a number line; Infinity is a set that may be countable or uncountable, depending on what kind of infinity you're working with.
There are infinities with higher cardinality than others. Infinity relates to set theory, and if you try to simply imagine it as a "position" in a line of real numbers, you'll understandably have an inconsistent mental model.
I highly recommend checking out Cantor's diagonal argument. Mathematicians didn't invent infinity as a curiosity; it solves real problems and implies real constraints. https://en.wikipedia.org/wiki/Cantor's_diagonal_argument
S is a function symbol. S(0) (in PA) is not a function. It is an expression involving one.
I was just saying that “one more than 0” isn’t a function just because in Peano arithmetic, the successor function, along with the constant 0, is used to denote natural numbers.
Diagonal argument doesn’t work in a constructive ground. It’s not a matter of whether the conclusion is valid, but if we have blind faith in the premises and are fine about speaking of something we can’t build.
They are things that humans will never be able to construct, no matter how far their control over the universe surrounding them might go. To start with, humans can create the universe, — whether it’s infinite or not.
This h=G(f) has the property that, for all i, there exists a j such that f(i)(j)≠h(j) . In particular, j=i will work for this.
It seems to me that this is all constructive.
The only out that I see is to not consider the class of “functions from N to {0,1}” to be something that exists (as a set, or type, or whatever).
Like, you can fairly reasonably hold the position that there is no powerset of the natural numbers, but you can’t reasonably hold the position that it exists and that there is a surjection from the natural numbers to it. (Likewise with any other set N. This isn’t specific to the natural numbers.)
We have a constructive refutation of that claim, in the sense that we have a construction of a function which, given such a surjection (as in, a function along with a promise that the provided function is such a surjection), produces a contradiction.
I claim the reason is that 5 is prime, while 10 is composite (10 = 5 times 2).
Therefore, 5 and 10, and 2, exist.
In any case existence of mathematical objects is a different meaning of existence to physical objects. We can say a mathematical object exists just by defining it, as long as it doesn't lead to contradiction.
You try answering the question without speaking of 5 or 10.
That is my argument.
The thing is assuming that 5 exists to conclude that 5 exists is obviously circular.
In particular, I would expect that if numbers don’t exist, the explanation I gave of the phenomenon I described, couldn’t be correct.
It's similar for the case of programs or algorithms. We can say that a sorting algorithm exists, or a chess-playing program or whatever, which means we know how to implement the logical process in some physical system, but it doesn't mean that they have some kind of existence which is independent of the physical systems. It's just a way of talking about patterns that can be common to many physical systems
I of course don’t mean that mathematical objects (such as the number 2, or some sorting algorithm) have the same kind of existence as my bed. To make the distinction, I would say that my bed “physically exists”.
Physical objects aren't like that because you can discover that they exist by empirical investigation.
In mathematics the discoveries are about the logical implications of sets of axioms. Some of those axioms contain assertions of existence, like a number 0 in Peano arithmetic or the empty set in set theory, and then you can prove statements about these objects based on the axioms. It's circular to infer from these conclusions that the axioms are true.
What's interesting is why certain axiom systems are so useful and fruitful. Personally I think it's because they evolved that way from our investigations of the physical world, but that's another matter
What you’ve pointed out is that the interactions of your cards, when confined to a particular set of manipulations and placements, is equivalent to a certain abstract model.
Reality does, stuff happens, etc. But physics is an abstract model we use to make predictions about reality — and triangles are part of that abstract model, not things that actually exist.
You can’t, for instance, show me a triangle. Just objects that are approximated by the abstract concept in physics.
Or in the other direction, if numbers stop at 3, that certainly won't falsify "every number above 7 is the sum of two other numbers". It will prove that it's true. And the extremely strong conjecture immediately proves that the extremely weak one is false.
(Granted, there are other objects that may seem ugly which can only be constructed by reference to infinite things.)
You can do a lot of things without assuming that the natural numbers keep going, but it is plain awful to work with.
I agree finitism and ultrafinitism are worth more development. Infinity is a hack in the sense that it effectively represents a program that doesn't halt, and when you start thinking of it this way, then all sorts of classical mathematical arguments start looking a little fishy, at least when applying math to the real world. For instance, I do think this is at the heart of some confusions and difficulties in physics.
It is very tempting to use it in place of a number, and so mathematicians (being humans) did that.
Yes, mathematicians hack too. Maybe they even invented it.
When mathematicians are working with infinite objects, it is not by plugging in "infinity" somewhere a number should go, in order to imagine that the rule that would construct an object if that were a number, constructs an object. No. Rather, (in a ZF-like foundations) the axiom of infinity assumes that there is a set whose elements are exactly the finite ordinals. (Or, assumes something equivalent to that.) From this, various sets are constructed such that the set is e.g. in bijection with a proper subset of itself (there are a handful of different definitions of a set being "infinite", which under the axiom of choice, are equivalent, but without AoC there are a few different senses of a set being "infinite", which is why I say "e.g.").
In various contexts in mathematics, there are properties relevant to that specific context which correspond to this notion, and which are therefore also given the name "infinite". For example, in the context of von Neumann algebras, a projection is called a "finite projection" if there is no strict subprojection that is Morray-von Neumann equivalent to it. Or, in the context of ordered fields, an element may be called "infinite" if it is greater than every natural number.
Usually, the thing that is said is that some object is "infinite", with "infinite" an adjective, not saying that some object "is infinity" with "infinity" a noun. One exception I can think of is in the context of the Surreal Numbers, in which the Gap between finite Surreal Numbers and (positive) infinite Surreal Numbers, is given the name "infinity". But usually objects are not given the name "infinity", except as like, a label for an index, but this is just a label.
I suppose that in the Riemann sphere, and other one-point compactifications, one calls the added point "the point at infinity". But, this kind of construction isn't more "make believe" than other things; one can do the same "add in a point at infinity" for finite fields, as in, one can take the projective line for a finite field, which adds a point that is doing the same thing as the "point at infinity" in the complex projective line (i.e. the Riemann sphere).
I think Baez's paper, Struggles with the Continuum, shows a lot of past difficulties we've had that resulted from this:
In a way, the axiom of infinity seems to behave much like other axioms that assert the existence of even larger mathematical "universes": it's worth being aware of what parts of a mathematical development are inherently dependent on it as an assumption, which is ultimately a question of so-called reverse mathematics.
https://en.wikipedia.org/wiki/Finitism
https://en.wikipedia.org/wiki/Constructivism_(philosophy_of_...