3 ms·

I don’t understand constructivism at all. No numbers are real. They are all an entirely abstract construction, like lines and planes and open sets and closed

I don’t understand constructivism at all. No numbers are real. They are all an entirely abstract construction, like lines and planes and open sets and closed balls and metric spaces and everything else. If I construct a number by saying it’s the limit of the sequence sqrt(2)-1/n as n->infinity that’s just as real as the number 1 or 1/2. It just seems really arbitrary to privilege one kind of construction over another. We want a complete ordered field, so we built the reals. Saying they don’t exist or they aren’t real or whatever seems just to be completely beside the point. They are real enough to do the thing we want them to do. They are just as real as anything else in maths.

futune 3mo ago
The numbers you mentioned are computable numbers. Constructivists and intuitionists have no problem with them, generally. The problem is that there is only a countable number of computable reals, so what do we do about all the other reals? The ones that nobody will give an example of because it is simply not possible to do so, which is to say, the vast majority of reals? Does it exist if it is impossible to show an example? Only if "exist" is interpreted to mean "you cannot deduce a contradiction from assuming them", which is a logically consistent position. But if you mean "they are in some sense actually there", in some describable way, then it's muddier. And you can build a logically consistent position from declaring "they are not there" as well. Precisely because no counterexample can be produced. And then there's ultrafinitists, and yeah, they are a bit bonkers.
- seanhunter 3mo ago
  They are in a very meaningful sense actually there. If I draw a curve I want the line not to have holes in it, and they have to be there for that to be true. More importantly, a function is its graph so if I want my functions to be continuous I need them to be there.
  - xscott 3mo ago
    No matter how small you go, between every two real numbers is a computable number, and between every two computable numbers is a real number that's not computable. If you restricted yourself to computable numbers, are you sure there are any holes in your graphs and functions? Can you point to or name one? :-)
    - seanhunter 3mo ago
      I understand the point about density and I'm fine with the concept of functions being continuous over a restricted domain (eg the rationals even) but if I draw a line and label one end 0 and one end 1 then I have plotted the set of all numbers in that interval, not just the ones we find computationally convenient. The historical context about the constructivist movement is that it was a religiously-inspired objection to the work of Cantor, who some random bishop said was challenging God with his work on transfinite numbers because God owned infinity. I just find it weird that now people try to pretend that it's somehow more rigorous when really it's just an alternative axiomatic perspective that started in this shonky way and has grown to a point where it's just about respectable.
      - xscott 3mo ago
        I can't speak to the religious bits or history. That's certainly not my motivation for thinking about this stuff. It's not about computational convenience either. Both of those seem like strawmen, but maybe they're relevant to other people. The problem to me is that the Reals which aren't computable are absurd. We've never used any of them in all history. We can only put names on a very special few, those are countable, and we don't even know their value very well. And the main way we prove that the Reals are uncountable is to use a proof by contradiction. It would take too long to spell it out, but they aren't really just contradicting "Reals are Countable". It's "All that other stuff we think is true AND Reals are Countable" that gets contradicted. Once you accept the Reals, the Axiom of Choice is not simply obvious any more. And if you go down that path you get things like the Banach-Tarski paradox. To me THAT ought to be a proof by contradiction that we've made a mistake somewhere. More interesting than that though: If the universe we live in requires non-computable Reals to describe it carefully, then it says something very weird about determinism. In order to compute a future state of a system, we need to use numbers we can't compute?!?

futune 3mo ago

The numbers you mentioned are computable numbers. Constructivists and intuitionists have no problem with them, generally. The problem is that there is only a countable number of computable reals, so what do we do about all the other reals? The ones that nobody will give an example of because it is simply not possible to do so, which is to say, the vast majority of reals? Does it exist if it is impossible to show an example? Only if "exist" is interpreted to mean "you cannot deduce a contradiction from assuming them", which is a logically consistent position. But if you mean "they are in some sense actually there", in some describable way, then it's muddier. And you can build a logically consistent position from declaring "they are not there" as well. Precisely because no counterexample can be produced. And then there's ultrafinitists, and yeah, they are a bit bonkers.

seanhunter 3mo ago
They are in a very meaningful sense actually there. If I draw a curve I want the line not to have holes in it, and they have to be there for that to be true. More importantly, a function is its graph so if I want my functions to be continuous I need them to be there.
- xscott 3mo ago
  No matter how small you go, between every two real numbers is a computable number, and between every two computable numbers is a real number that's not computable. If you restricted yourself to computable numbers, are you sure there are any holes in your graphs and functions? Can you point to or name one? :-)
  - seanhunter 3mo ago
    I understand the point about density and I'm fine with the concept of functions being continuous over a restricted domain (eg the rationals even) but if I draw a line and label one end 0 and one end 1 then I have plotted the set of all numbers in that interval, not just the ones we find computationally convenient. The historical context about the constructivist movement is that it was a religiously-inspired objection to the work of Cantor, who some random bishop said was challenging God with his work on transfinite numbers because God owned infinity. I just find it weird that now people try to pretend that it's somehow more rigorous when really it's just an alternative axiomatic perspective that started in this shonky way and has grown to a point where it's just about respectable.
    - xscott 3mo ago
      I can't speak to the religious bits or history. That's certainly not my motivation for thinking about this stuff. It's not about computational convenience either. Both of those seem like strawmen, but maybe they're relevant to other people. The problem to me is that the Reals which aren't computable are absurd. We've never used any of them in all history. We can only put names on a very special few, those are countable, and we don't even know their value very well. And the main way we prove that the Reals are uncountable is to use a proof by contradiction. It would take too long to spell it out, but they aren't really just contradicting "Reals are Countable". It's "All that other stuff we think is true AND Reals are Countable" that gets contradicted. Once you accept the Reals, the Axiom of Choice is not simply obvious any more. And if you go down that path you get things like the Banach-Tarski paradox. To me THAT ought to be a proof by contradiction that we've made a mistake somewhere. More interesting than that though: If the universe we live in requires non-computable Reals to describe it carefully, then it says something very weird about determinism. In order to compute a future state of a system, we need to use numbers we can't compute?!?