disregardthat said:
I'm not sure we are on the same wavelength here (and I'm not particularly comfortable talking about constraints)
The source of the discomfort is probably because "constraints" are implicitly active and causal. So it goes against the spirit of reductionism where things either are, or they aren't, there is no need to limit things so that they actually are just "are", and the other things are in fact "aren't".
But the framing of an axiom is an active constraint on possibility. It is the mathematician saying: many things might be true, but I am asserting now that this precise thing is true (and so everything else follows).
, but I have been talking about mathematical reality as in platonic reality, not as in physical reality. Mathematical statements are never statements about the physical reality
Logical statements are neither statements about physical reality, but in mathematics we have in a much more radical sense not really "statements" at all compared to logical statements and statements of science. Statements, theorems as well as axioms, can be (perhaps more aptly) be considered as rules, and are not statements about anything.
You are saying that mathematics is just the syntax. The semantics is unnecessary. Whereas science and logic need something substantial(!) to ground their formal statements.
I agree that mathematics is pretty much just concerned with the development of correct syntactical operations. It seems to be quite removed from the messy business of real things. But still, mathematics must make reality (or at least our measurable impressions of reality) both its point of departure and also its eventual place of return.
So to get the enterprise of rule-making, etc, going - the exploration of the rich syntactical possibilities inherent in any chosen set of axioms - the axioms have to be formed. And what it seems safe to presume is something humans agree from discussing their collective experience of reality. Axioms may be our sharp departure point from reality, but they arise out of that reality (or our ontic beliefs) by the same token.
Then having elaborated itself in the platonic realm of pure ideas thinking themselves

, mathematics must return to reality as modelling. There is a reason why human society values maths and it is not because there is something useful in endless syntactical noodling. Rather, it is Wigner's unreasonable effectiveness that makes maths valued. Exactly what kind of syntax will be useful is unpredictable (and much may indeed be useless), but the payoff in terms of being able to model reality is obvious enough.
Take set theory. We talk about sets, of course. But we never define sets. In fact, we don't need a definition of sets. It is not only useless, but irrelevant for the mathematics which spawn from it (and it would even be futile to try to do so in set theory). We have ourselves simply a collection of rules to utilize. It is actually a very odd thing to say that set theory is "a theory of sets". "Sets" are not really something that is being talked about (mathematically that is, we can casually casually talk about sets outside the formalities of mathematics). Sets did not get their existence the moment we created set theory, and sets did not exist before we invented set theory (platonically). Mathematically they exist, but mathematical existence, as I mentioned before, might as well be regarded as another rule.
I'm a bit confused here as I would have thought the story was that mathematics attempted to find a foundations in set theory. Then when that didn't work, it had to go looking for something more general (less constrained) in category theory.
To "exist" in the platonic mathematical sense, set theory would seem to have to be "self-evident" in some incontrovertible fashion. Just as reality is self-evident and resists our attempts to controvert its existence.
Set theory couldn't prove itself. And I would have thought all the business with Russell and Godel was evidence that maths isn't actually platonic and needs reality as its at least vestigal departure point.
Also, in saying things like ideas exist "before" we think them, well they exist as concrete possibilities rather than actually existing. That would be the realist position. The platonic position would indeed say that the ideas exist outside of time itself. They are immutable (changeless) and so therefore "eternally present..or not present" of necessity. It is truth that creates a mathematical idea, and the untrue idea cannot exist.
This platonic statement sounds convincing. Until you come back to the fact that it all has to start somewhere. That in fact our axioms, our self-evident truths, are rooted in our very human impressions of reality. Mostly this fact can be avoided as people rarely discuss axioms in a philosophical way. They just assume them and get on with the game of syntactical elaboration.
I'm not entirely sure how to understand what you say about reality having a topology, but it must be clear as day that the physical nature can not have mathematical properties in any fundamental fashion. Mathematics will in this sense only serve as a tool in a scientific modeling nature (in which it will make sense to talk about the topology or the geometry of space). My point is really that space does not have a topology which we attempt to describe mathematically.
What I said was that geometry/topology has been generalised to the point where it no longer tries to describe our reality, but describes any kind of "world" as a somehow connected space or set or relations. Then to use the mathematics to describe/model our own world, we have to add back some of the constraints that have been relaxed.
So the real world has organisation. It has particular global constraints that exist! They may have developed, they may be still dynamic and slowly changing, but they are definite and persistent enough that they seem to define our universe. In maths, we have stripped away everything that seems particular so as to arrive at the most general. And so to model reality, we have to do the (unnatural, artificial) thing of adding constraints back to simulate the actual organisation of reality.
OK, says our mathematician God, I need to construct me a world. Give me just three spatial dimensions. Toss in a few more perhaps to make some stringy particles. Let's inflate this thing so big its got to look largely flat. I need a few constants and a big entropy gradient. And dang, I baked me a universe.
Reality itself would have arisen quite differently - not constructed by some unconstrained being but self-organised via the development of a particular set of constraints. And as I say, that self-organising story is tough to model because that is not the mental tool-kit we have been developing the past 2500 years.
You might actually need a maths that is a bit different in spirit. One that can model the development of global constraints rather than one where the mathematician stands outside and tosses constraints into the cooking pot to see what happens.