Re 1: Discussing and guiding the desirable theorems for general-purpose programs has been a major challenge for us. Proofs for their own sake (bad?) vs glorious general results (good but hard?). Actual human guidance there can be critical there at least for now.

Mind linking the experiment? Sounds interesting.

Classical logic has plenty of limitations/roadblocks, all logics do. Logic isn't a unified domain, but an infinite beach of structural shards, each providing a unique lens of study.

Classical logic was rejected in computer science because the non-constructive nature made it inappropriate for an ostensibly constructive domain. Theoretical mathematics has plenty of uses to prove existences and then do nothing with the relevant object. A computer, generally, is more interested in performing operations over objects, which requires more than proving the object exists. Additionally, while you can implement evaluation of classical logic with a machine, it's extremely unwieldy and inefficient, and allows for a level of non-rigor that proves to be a massive footgun.

As far as I understand it, classical mathematics is non-constructive. This means there are quite a few proofs that show that some value exists, but not what that value is. And in mathematics, a proof often depends on the existence of some value (you can't do an operation on nothing).

The thing is it can be quite useful to always know what a value is, and there's some cool things you can do when you know how to get a value (such as create an algorithm to get said value). I'm still learning this stuff myself, but inuitionistic logic gets you a lot of interesting properties.

In constructive logic, a proof of "A or B" consists of a pair (T,P). If T equals 0, then P proves A. If T equals 1, then P proves B. This directly corresponds to tagged union data types in programming. A "Float or Int" consists of a pair (Tag, Union). If Tag equals 0, then Union stores a Float. If Tag equals 1, then Union stores an Int.

In classical logic, a proof of "A or not A" requires nothing, a proof out of thin air.

Obviously, we want to stick with useful data structures, so we use constructive logic for programming.

There are non-computational interpretations of intuitionistic logic too, like every single thing mentioned on this page: https://ncatlab.org/nlab/show/synthetic+mathematics

I think stuff like "synthetic topology", "synthetic differential geometry", "synthetic computability theory", "synthetic algebraic geometry" are the most promising applications at the moment.

It can also find commonalities between different abstract areas of maths, since there are a lot of exotic interpretations of intuitionistic logic, and doing mathematics within intuitionistic logic allows one to prove results which are true in all these interpretations simultaneously.

I'm not sure if intuitionism has a "killer app" yet, but you could say the same about every piece of theory ever, at least over its initial period of development. I think the broad lesson is that the rules of logic are a "coordinate system" for doing mathematics, and changing the rules of logic is like changing to a different coordinate system, which might make certain things easier. In some areas of maths, like modern Algebraic Geometry, the standard rules of logic might be why the area is borderline impenetrable.

Excluded-middle `true` means "[provable] OR [impossible to disprove]".

Intuitionist/Constructivist `true` means, "provable".

The question you are asking determines what answers are acceptable.

Why build an airplane, if you already know it must be possible?

For ideological reasons.

Oops, just edited. I'm still fairly new to this area, so I keep mixing up my terms :)

More generally, any negative statements can be proven classically, even in intuitionistic logic. Intuitionistic logic does not have the De Morgan duality found in classical logic, you have to go to linear logic if you want to recover that while keeping constructivity. (The "negative" in linear logic actually models requesting some object, which is dual to constructing it. The connection with the usual meaning of "negative" in logic involves a similar duality between "proposing" a proof and "challenging" it.)

So proof by contradiction proves ¬p, but it requires the law of excluded middle to prove p (in the case of ¬p -> F)? I didn't realize that was constructive in the first case.

I think the most surprising thing I've learned taking formal math in college is just how much mathematicians are pragmatists (at least for my teacher with sample size n=1). They're much more interested in new ways to think about ideas, with a side effect of proofs for correctness. The proof is more about explaining why something works, not that it does.

I'm going to take a formal logic class in the fall, and my professor said something akin to "definitely take it if you're interested, just be aware that it probably won't come in use in most of the mathematics done today." The thing is the foundations are mostly laid, and people are interested in using said foundations for interesting things, not for constantly revisiting the foundations.

I think this is one reason most mathematicians don't see a need for formal proof assistants, since from their perspective it's one very small part of math, and not the interesting one.

This is not to say that proof assistants are a dead end—I find them fascinating and hope they continue to grow—but there's a reason that they haven't had a ton of traction.

Mathematicians use logic to talk about the mathematical world. But logic is not the world.

The link is exactly what I’m saying. I only hear cs people talk about it.

For mathematicians a proof is a means to an end, or a medium of expression - they care about what they say and why.

The correspondence isn’t about C programs corresponding to proofs in math papers. It’s a very a specific claim about kinds of formal systems which don’t resemble how math or programming is done.

> one of the most important living

Maybe. But more clearly one of the most popular online.