Comment by fmap

> For this particular question we have a great answer in the form of homotopy type theory. It handles all the complications that the author mentions. This is one of the reasons type theorists were excited about HoTT in the first place.

Arguably, Homotopy type theory still doesn't solve the problem, because while it strengthens the consequences of isomorphism it doesn't distinguish between "isomorphism" and "canonical isomorphism", whereas (some?) mathematicians informally seem to think that there's a meaningful difference.

> The only problem is that the author is working in Lean and apparently* dismissive of non-classical type theory. So now we're back to lamenting that equality in type theory is broken...

In my opinion, Lean made a number pragmatic choices (impredicative Prop, non-classical logic including the axiom of global choice, uniqueness of identity proofs, etc.) that enable the practical formalization of mathematics as actually done by the vast majority of mathematicians who don't research logic, type theory, or category theory.

It's far from established that this is possible at all with homotopy type theory, yet alone whether it would actually be easier or more economical to do so. And even if this state of affairs is permanent, homotopy type theory itself would still be an interesting topic of study like any other field of mathematics.

> It handles all the complications that the author mentions. This is one of the reasons type theorists were excited about HoTT in the first place.

It's not that simple, Buzzard actually seems to be quite familiar with HoTT. The HoTT schtick of defining equality between types as isomorphism does 'handle' these issues for you in the sense that they are treated rigorously, but there's no free lunch in that. You still need to do all the usual work establishing compatibility of values, functions etc. with the relevant isomorphisms, so other than the presentation being somewhat more straightforward there is not much of a gain in usability.

Can you elaborate more on how you think HoTT can solve the issue Buzzard mentions that when formalizing informal mathematics, the author frequently has a model (or, a particular construction) in mind and requires properties present in the model, and then claims that this is true for all "canonically equivalent" objects without actually spending time explicating the abstract nonsense required to properly specify the properties that canonically equivalent objects preserve, for their developement? (see: a product vs. the product)

Edit: and furthermore, the situation where the obvious choice of universal property (read: level of isomorphism) was a poor one, in their attempt to formalize localizations.

I think 99.9% of the people talking about HoTT haven't used HoTT in practice. They just see isomorphism is equivalent to equivalence(or something similar), comes up with a mental model and think this is the main contribution of HoTT, whereas isomorphism is a well studied thing even before formal set theory was a thing. HoTT gets weird and anyone who has tried any tool to prove anything in HoTT knows this, and this is the reason why it didn't got adopted even though many leading tools like lean worked hard to implement it.

For reference, the paper mentions the term "homotopy" exactly once, in this sentence:

> The set-theoretic definition is too strong (Cauchy reals and Dedekind reals are certainly not equal as sets) and the homotopy type theoretic definition is too weak (things can be equal in more than one way, in contrast to Grothendieck’s usage of the term).

Classical versus non-classical has nothing to do with it. HoTT does not work to capture what a mathematician means by "unique isomorphism" because witnesses of equality in HoTT are very much not unique.

I think HOTT helps remove a lot of the footguns and computational hairiness around using type-theoretic mathematics. But the issues Buzzard is talking about are really more profound than that. This relates to a much deeper (but familiar and nontechnical) philosophical problem: you assign a symbol to a thing, and read two sentences that refer to the same symbol ("up to isomorphism"), but not necessarily the same thing. If the sentences contradict each other, is it because they disagree on the meaning of the symbol, or do they disagree on the essence of the thing? There is a strong argument that most debates about free will and consciousness are really about what poorly-understood thing(s) we are assigning to the symbol. Worse, this makes such debates ripe for bad faith, where flaws in an argument are defended by implicitly changing the meaning of the symbol.

In the context of """simple""" mathematics, preverbal toddlers and chimpanzees clearly have an innate understanding of quantity and order. It's only after children fully develop this innate understanding that there's any point in teaching them "one," "two," "three," and thereby giving them the tools for handling larger numbers. I don't think it makes sense to say that toddlers understand the Peano axioms. Rather, Peano formulated the axioms based on his own (highly sophisticated) innate understanding of number. But given he spent decades of pondering the topic, it seems like Peano's abstract conception of "number" became different from (say) Kronecker's, or other constructivists/etc. Simply slapping the word "integer" on two different concepts and pointing out that they coincide for quantities we can comprehend doesn't actually do anything by itself to address the discrepancy in concept revealed by Peano allowing unbounded integers and Kronecker's skepticism. (The best argument against constructivism is essentially sociological and pragmatic, not "mathematically rational.")

Zooming out a bit, I suspect we (scientifically-informed laypeople + many scientists) badly misunderstand the link between language and human cognition. It seems more likely to me that we have extremely advanced chimpanzee brains that make all sorts of sophisticated chimpanzee deductions, including the extremely difficult question of "what is a number?", but to be shared (and critically investigated) these deductions have to be squished into language, as a woefully insufficient compromise. And I think a lot of philosophical - and metamathematical - confusion can be understood as a discrepancy between our chimpanzee brains having a largely rigorous understanding of something, but running into limits with our Broca's and Wernicke's areas, limits which may or may not be fixed by "technological development" in human language. (Don't even get me started on GPT...)

The fact that Buzzard is writing this paper at all to me shows a much higher degree of respect than before --- see how humble the overview is. I think this is great progress!