When you've counted up to and including n, there's the subsequent number n+1, that you have not yet counted. Yet for the set of *all* natural numbers to exist it would have to be possible to finish counting the natural numbers, beginning from 0. One could counter this by saying that the set of all natural numbers exists even though it is impossible to construct. But that would be a highly dubious assumption, and a theory built on such an assumption would be a joke.
the fact that you can always go n + 1 is almost literally the definition of a countably infinite set
so your argument is, in a way, self-contradictory. You dont like an infinite set because there's always an n + 1, yet any set where you
cannot go n + 1 is a finite set. I.e. you're almost arguing it should be illegal to add 1 to a number once it gets too high.
not trying to be argumentative, just thought it was an amusing thought
a lot of definitions of infinity in mathematics can be though of as just that - actions that can be repeated without end. For example I can divide a number by 2 as many times as I like; there's no final number where I am forced to stop. Thus the sequence of these divided numbers is an infinite set. The entirety of calculus is based on this idea.
There's a 2000 year history in the philosophy of mathematics of making a distinction between actual and potential infinities ("potential infinity" is kind of a misleading name though, because such an "infinity" isn't really infinite), and for most of that history mathematicians accepted potential infinities, but rejected actual ones. The latter largely changed in the aftermath of Cantor's work on set theory. Basically all mathematicians now accept the existence of both.
You seem to have failed to appreciate the distinction between the two concepts. Or it may be that we don't share some presuppositions about something like what it means for a mathematical object to exist.
I reject the existence of actual infinities, but accept the existence of potential ones (some mathematicians even reject the existence of potential infinities, and so would indeed claim that there is a biggest number or at least withhold judgement as to whether there is one. And that's not as crazy as one might think because it's a sort of down to earth attitude they have. "It may not be physically possible to construct just any number."
"The fact that you can always go n + 1 is almost literally the definition of a countably infinite set
so your argument is, in a way, self-contradictory. You dont like an infinite set because there's always an n + 1, yet any set where you cannot go n + 1 is a finite set. I.e. you're almost arguing it should be illegal to add 1 to a number once it gets too high."
If I were to interpret what you said charitably, I would assume what you mean by "can always go n+1" is that for any n in the set, n+1 is also in the set, and not that you can always add another element n+1 to the set (because you can't add an element to any set, whether it's a finite set or a supposedly infinite set). But the problem for you is that given the first interpretation, it's actually harder to see where the supposed self-contradiction lies.
The fact that I accept that the process of trying to count the natural numbers is unlimited, does not mean that I have to accept that there exists a countably infinite set like the set of all natural numbers. Let's get into some details. The set of all natural numbers can be extracted from the axiom of infinity - which says: "A set exists that contains the empty set and the successor set to every element in the set. The successor set to a set A is the union of A and the set containing just A." The axiom implicitly gives us rules for generating lists of sets, and each set can be defined to be a unique natural number: "If you haven't jotted down the empty the set, then do so, otherwise keep jotting down the successor set to the set you previously jotted down":
{}=0
{0}=1
{0,1}=2
{0,1,2}=3.
...
The process above is an example of a potential infinity because it can generate an unlimited number of lists and lists unlimited in size, but the process is never complete, each list is created in a finite number of steps. And so a list can never be made infinitely long this way nor will one ever create anything but a finite number of them.
I accept that the process is unlimited, and so I accept the existence of potential infinities. Someone who rejects the existence of potential infinities would say the process has a limit (or perhaps something else) or that they merely are not convinced that it is unlimited.
What I reject about the axiom of infinity is that despite the impossibility of generating an infinitely long list using the two rules mentioned above, it basically implies that there
already is such an infinitely long list and claims that a set exists which contains all the sets in that infinitely long list.
a lot of definitions of infinity in mathematics can be though of as just that - actions that can be repeated without end. For example I can divide a number by 2 as many times as I like; there's no final number where I am forced to stop. Thus the sequence of these divided numbers is an infinite set. The entirety of calculus is based on this idea."
Here you're presupposing that there already is an infinite sequence like a, a/2, a/4, a/8... which forms a
completed totality and exists as a given object - an actual infinity.
The notion that such an object exists does not follow from the fact you can begin a sequence with a, a/2, a/4 and keep going without any limit - which is a potential infinity.
An unlimited process can
not be
completed, so why would it follow that such an object exists?
Edit: I'll try to make a steel man argument here, which is seemingly plausible (maybe?) but makes a conceptually flawed implicit assumption:
If you can keep jotting down rational numbers without any limit, then there can't be a finite number of rational numbers, because if there were, you'd eventually have to stop because you would run out of numbers to pick from.
So there is an infinite set of them.
Here's the response: there not being a finite number of rationals does not mean there's an infinite set of them.
There isn't a finite number of rational numbers, but that's because there is no fixed number of them. Consistent with that I would simply say that the rational numbers are potentially infinite: they are generated through an unlimited process.