Comment by IngoBlechschmid | Hacker News Reader

Comment by IngoBlechschmid | Hacker News Reader

IngoBlechschmid

The article provides a nice introduction to the subtleties surrounding the axiom of choice. The last paragraph briefly mentions that there are other flavors of mathematics in which the axiom of choice doesn't hold. For people who are intrigued by the idea of different flavors of mathematics, but all still set-based, a couple months ago I wrote an expository summary of the so-called "multiverse philosophy in set theory". You can find it at https://iblech.gitlab.io/bb/multiverse.html, and I'm happy to answer any questions or comments you might have.

There are also even wilder flavors of mathematics which are not set-based. In those we can have various dream results which directly contradict the mathematical canon (but are internally consistent and have a certain precise relation to the ordinary mathematical world). For instance we can have that every function is computable by a Turing machine, that every real function is continuous or that the reals include infinitesimal numbers. An introduction to these flavors, aimed at philosophers of mathematics, can be found here: https://rawgit.com/iblech/internal-methods/master/paper-film...

>The only caveat of Theorem 1.3 is that toposes generally only support intuitionisticreasoning, not the full power of the ordinaryclassical reasoning.

Why is intuitionistic reasoning blessed as the "universal reasoning," when there are other ways of doing logic that permit even fewer inference rules?

Good question! I would disagree that intuitionistic reasoning is blessed as the "universal reasoning". As you say, there are other even weaker but still useful systems around.

It is just a fact of life, without any room for philosophical preferences, that the largest common denominator of all toposes is exactly intuitionistic reasoning, not more, not less.

But there are, besides toposes, also other kinds of mathematical structures which can be regarded as mathematical universes! And the largest common denominator of those other kinds can be more or less than intuitionistic reasoning.

Going up, we have for instance models of ZFC. By definition, their largest common denominator is ZFC, so more than intuitionistic reasoning.

Going down, we have the so called "arithmetic universes". Their largest common denominator is "arithmetic type theory", a predicative flavor of intuitionistic reasoning. (To a very rough first approximation which doesn't at all do justice to this intriguing topic, "predicative" means "no powerset axiom". The terminological convention is that by default, "intuitionistic reasoning" refers to impredicative intuitionistic reasoning.)

And then there are a couple other kinds still.

That said, toposes with their impredicative intuitionistic reasoning do occupy a sweet spot. They are sufficiently general to yield useful applications in several branches of mathematics while not being too general.

At the beginning of "multiverse.html" you say "In the beginning of the 19th century..."

I think you mean 20th.

That was a fascinating read; thank you. Not every day that I learn about a whole new important multiverse!

Reply on news.ycombinator.com