# Open Thread July 2018

post by toonalfrink · 2018-07-10T14:51:12.351Z · LW · GW · 9 commentsFor those low-effort ideas that are worth a little attention but perhaps not a lot (yet).

(shouldn't a bot be posting these?)

Prompts for things to say:

- What accomplishments are you celebrating from the last month?
- What are you reading?
- What subject has taken up most of your thoughts lately?
- What is something your fellow rationalists don't understand, but should?

## 9 comments

Comments sorted by top scores.

What a wave of nice technical posts today! Since it seems like many of them are getting zero comments, I'd like to encourage everyone to comment on each other's posts, not just watch your own.

I was hoping there would be some kind of welcome thread for doing this, but I suppose an open thread is as good a place as any to introduce myself. I call myself Liskantope online, in real life I'm a postdoctoral researcher in pure math, but I like to blog on Wordpress and Tumblr under this handle. I identify loosely with the online rationalist movement and would like to become more involved in some ways, including participating at least a little on LessWrong. I don't have much background in either AI or EA stuff, but I'd like to absorb more knowledge or at least intuition in these areas.

An accomplishment of the past month that I'm celebrating? I spoke at a conference and it went really well, not often I get the chance to do that and I hope giving that talk has set off a positive feedback loop where I will get more recognition which leads to more speaking opportunities. Success in academia relies too much on self-perpetuating recognition or lack thereof for my liking.

Could logical inductors be used as a partial solution to Hilbert's Second Problem (of putting mathematics on a sure footing)? Thanks to Gödel we know that there are lots of things that any given theory can't prove. But by running a logical inductor we could at least say that these things are true with some probability. Of course a result proved in the "Logical Induction" paper is that the probability of an undecidable statement tends to a value that is neither 0 or 1, so we can't use this approach to justify belief in a stronger theory. But I noticed a weaker result that does hold. There's a certain class of statements such that (assuming ZF is consistent) an inductor over PA will think that they're very likely as soon as it finds a proof for them in ZF.

This class of statements is those with only bounded quantifiers; those where every "∀" and "∃" are restricted to a predefined range. This class of statements is decidable, meaning that there's a Turing machine that will take a bounded sentence and will always halt and tell you whether or not it holds in *ℕ*. Because of this every bounded sentence has a proof (or a proof of its negation) in both PA and ZF (and PA and ZF agree which it is).

But the proofs of a bounded sentence in PA and ZF can have very different lengths. Consider the self-referential bounded sentence "PA cannot prove this sentence in fewer than 1000000 symbols". This must have a proof in PA, since we can just check all sentence with fewer than 1000000 symbols by brute force, but its proof must be longer than 1000000 symbols, or else we would get a contradiction. But the preceding sentences constitute a proof in ZF with much fewer than 1000000 symbols. So the sentence is provable in both PA and ZF, but the ZF proof is much shorter.

It might seem like the bounded sentences can't express many interesting concepts. But in fact I'd contend that they can express most (if not all) things that you might actually need to know. For example, it seems like the fact "For all x and y, x + y = y + x" is a useful unbounded sentence. But whenever you face a situation where you would want to use it there are always some particular x and y that apply in that situation. So then we can use the bounded sentence "x + y = y + x" instead, where x and y stand for whichever values actually occurred.

Now I'll show that logical inductors over PA eventually trust proofs in ZF of bounded sentences (assuming ZF is consistent). Consider the Turing machine that takes as input a number n and searches through all strings in length order, keeping track of any that are a ZF proof of a bounded sentence. When it's been searching for n timesteps it stops and outputs whichever bounded sentence it found a proof for last. Call this sentence ϕ_n. Now let P be a logical inductor over PA. Assuming that ZF is consistent, the sequence ϕ_n are all theorems of PA, and by construction there's a polynomial time Turning machine that outputs them. So by a theorem in the logical inductor paper, we have that P_n(ϕ_n) tends to 1 as n goes to infinity, meaning that for large n the logical inductor becomes confident in ϕ_n sometime around day n. If a bounded statement ϕ has a ZF proof in m symbols then it's equal to ϕ_n for n ~ exp(m). So P begins to think that ϕ is very likely from day exp(m) onward.

Assuming that the logical inductor is working with a deductive process that searches through PA proofs in length order, this can occur long before the deductive process actually proves that ϕ is true. The exponential doesn't really make a difference here, since we don't know exactly how fast the deductive process is working. But it hardly matters, because the length of ZF proofs can be arbitrarily better than those in PA. For example the shortest proof of the sentence "PA cannot prove this sentence in fewer than exp(exp(exp(n))) symbols" in PA is longer than exp(exp(exp(n))) symbols, whereas the length of the shortest proof in ZF is about log(n).

So in generality what we have proved is that weak systems will accept as very good evidence proofs given in stronger theories, so long as the target of the proof is a bounded sentence, and so long as the stronger theory is in fact consistent. This is an interesting partial answer to Hilbert's question, since it explains why we would care about proofs in ZF, even if we only believe in PA.

Well, your argument assumes that ZF is consistent, so it wouldn't convince someone who only believes in PA.

Sure, the inductor doesn't know which systems are consistent, but nevertheless it eventually starts believing the proofs given by any system which is consistent.

In general, are we also encouraged to ask questions here?

I have one about layout: is the page width of posts the same for everyone, given that the window is wide enough? That would mean everyone has line breaks etc at the same positions.

Note that it wouldn't mean that, since there are various other factors that might affect it. E.g. people might set their fonts larger. It might mean that almost everyone has line breaks in the same place. Though even then you're assuming that phone browsers are wide enough or uncommon enough, and that greater wrong is uncommon.

Is there a preferred way to flag spam posts like this one: https://www.lesswrong.com/posts/g7LgqmEhaoZnzggzJ/teaching-is-everything-and-more ?