## Thursday, June 18, 2009

The Importance of Uncomputable Models

Inspired by this blog.

I have mentioned most of these thoughts before, but I am writing them up in one post as a cohesive argument. I will argue that uncomputable models are important. I am not arguing that people think in ways that computers cannot, but rather than people and computers alike can benefit from using models that have "holes" which represent meaningful questions that can't be answered definitively by running a computation.

The world that we live in consists of a bunch of macro-objects. What are macro-objects? Roughly speaking, they are coherent structures of micro-objects, which recur in both time and space.

A macro-object "emerges" from the micro-dynamics. It is a structure which persists, propagating itself into the future, like a wave in water.

A spherical bubble is a macro-object, because slight warps in the sphere get evened out over time. (Until it pops.)

Similarly, a water droplet, a planet, and a star.

An atom is an emergent object (though not macro-level) because the positive and negative charges of the electrons and protons enter into a stable relationship.

A grasshopper is a stable entity because its metabolism maintains a homeostasis which allows it to live for a fair amount of time.

Similarly for all living organisms.

And, so on.

The key idea here is that all of these objects are in a convergent state: a state that (within limits) it always returns to after wandering away from.

Now, to logical foundations. The halting problem is unsolvable; there is no computable algorithm which can tell you which computations do and do not halt. But, suppose we could run our computer an infinite amount of time to get the answer. A machine that can do this is called a hypercomputer. Then, we could solve the halting problem by waiting to see if the computation in question halted; so, we can use hypercomputations to answer many questions that regular computations cannot answer. However, we've got a new type of problem. Some computations will flip back and forth between answers infinitely often, so that when we run them an infinite amount of time, the output is indeterminate. The result of the hypercomputation is then undefined, or "nonconvergent".

Asking whether a hypercomputation converges is analagous to asking whether a normal computation halts. In a specific sense, it is twice as hard: if we solved the halting problem for normal computations, and made a little magic box that can give the correct answer for halting questions, and connect that to an ordinary computer, then we have a machine equivalent to a hypercomputer. Asking whether the programs of the new machine halt is actually equivalent to asking if hypercomputations converge.

So, halting is uncomputable, but convergence is doubly so!

Yet, I have argued that convergence is all around us. On an everyday basis, we deal with convergent states as if they were solid entities. So, I argue, we are viewing the world through an uncomputable model.

Mathematically, one should expect reasoning about convergence to be quite hard (assuming, as I do, that human reasoning is computable). Everyday reasoning is not "quite hard" in this way. We mitigate the full force of the uncomputability of our models with many coping strategies; we mainly reason under the assumption of convergence (for structures that have converged in the past), rather than attempting to question this assumption. We have to learn when things converge and fail to converge. Yet, even so, using uncomputable models is easier than trying to apply computable models to the problem. Asking whether a structure is generally convergent is a very useful abbreviation, approximately summing up a lot of questions about the state at a given time.

Also, it is important to admit that the mathematically pure concept of convergence is not quite what we are interested in. In practical situations, we are interested in whether something is quickly convergent. This is not uncomputable; however, it can be more expensive to check then reasoning abstractly about convergence. So (and this is probably the weakest point in my argument) I think it is worthwhile to keep reasoning about the uncomputable models.

Another interesting point is that, much of the time, while we have a fairly good concept of the emergent convergent objects we deal with day to day, we do not have such a good idea of the underlying dynamic. This means that, in practice, we do not ask too many convergence-hard questions. Often, we think we already have those answers, and instead we ask what sort of underlying structure might give rise to them.

PS--

I am being somewhat messy here, because "convergent" in the case of hypercomputation does not mean quite the same thing as "convergent" in the case of emergent objects. For one thing, convergence of entities has to do with correcting for disturbances from the external environment, while convergence for hypercomputations does not. I think this difference does not harm the argument. As I see it, emergent-convergence is a more general problem, having hypercomputation-convergence as a subproblem.

## Sunday, June 14, 2009

What They Don't Tell You About Type Checking

Typed lambda calculus
is not Turing-complete.

There. I said it.

More specifically, simply typed lambda calculus is not Turing-complete, and neither are any variants that are both strongly normalizing and have decidable type-checking. This is because programs that the type-checker verifies are guaranteed to compute a result. If such a type-checker allowed a Turing-complete set of programs, it would be a solution to the halting problem!

Really, I should have put two and two together earlier on this one. I suppose this is what comes of picking lambda calculus up by reading diverse things in diverse places rather than learning it from one authoritative source.

What this indicates for me is that, at least in many cases, the point of allowing more and more sophisticated type systems is to get closer to a Turing-complete system. That is why people add things like parametric polymorphism, dependent types, and kinds. When we add these to typed lambda calculus, it doesn't just get "more expressive" in the sense that a high-level programming language is more expressive than machine code; it is literally able to do things that a simpler type system could not.

This doesn't mean that strongly typed programming languages are not Turing-complete. Typically the type-checkers for these will not guarantee that the program contains no infinite loops. So, one must be careful to figure out exactly what one is dealing with.

## Wednesday, June 10, 2009

Sets and Truth

In this post, I will explain a way to create a theory of sets, given a theory of truth. These two foundational issues are typically treated as separate matters, so asking for a particular relationship to hold between a theory of truth and a set theory adds some interesting constraints to the situation.

(Part of this post is an edited version of an email I sent originally to Randall Holmes.)

Claim: A set is essentially a sentence with a hole in it. Set membership is characterized by the truth/falsehood of the sentences when we fill the holes.

The major justification for this way of understanding sets is the way we use the comprehension principle to talk about sets. The comprehension principle is what allows us to say, in a set theory, that if we can describe the membership requirements for a set then that set exists. For example, I can describe the requirement "it must be a prime number that is over 5 digits long", so the set of prime numbers over five digits long exists. (The comprehension principle with no restrictions leads to naive set theory, however, which is paradoxical.)

This view is not far from the way Frege explained sets, as I understand it. However, he distinguished the set as the extension of the sentence-with-hole; meaning, the things for which it is true.

So, suppose we've got a logic with enough machinery to represent computable functions, and furthermore we've got a description of the language in itself (ie, Godel-numbering-or-something-equivalent). Furthermore, we've got some theory of truth. Then, via the claim, we can already talk about sets, even though they haven't been purposefully introduced. In particular, "x is an element of y" is
interpreted as:

"When "x" is used to fill in the partial sentence Y, the result is true"

where "x" is the name of the term x (Godel-numbering-or-equivalent, again, for those who are familiar with such things), and Y is the sentence-with-hole corresponding to the set y.

The truth predicate is needed here in order to assert the result of the substitution. With the naive theory of truth, it is always meaningful to apply the truth predicate to a sentence. So, the naive theory of truth gives us the naive theory of sets, in which set-membership is meaningful for any set we can describe. Of course, this is inconsistent under classical logic.

So, what I'm saying is: if the claim is accepted, then the set theory is pinned down completely by the theory of truth. The naive theory of truth gives us the naive theory of sets. A tarski-hierarchy theory of truth gives us something vaguely resembling type theory. Kripke's theory of truth gives us a theory in which all sets exist, but not all membership evaluations are meaningful. In particular, Russel's set "all sets that do not contain themselves" exists. We can meaningfully say that any set of integers is in Russel's set, and that the set of all sets (which exists) is not. The paradoxical situation, in which we ask if Russel's set is a member of itself, is simply meaningless.

So good so far. But, there is the issue of extensionality to deal with. The axiom of extensionality is taken as a very basic fact of set theory, one that not even nonstandard set theories consider rejecting. Given the above discussion, however, the axiom of extentionality would be false. Two different sentences-with-holes can be logically equivalent, and so have the same extension. For example, "_ is an even prime number" and "_ added to itself equals 4" are the same set, but they are different sentences.

My solution here is to interpret the notion of set-equality as being a notion of logical equivalence between sentences-with-holes, rather than one of syntactic equivalence. In other words, "x=y" for two sets x and y needs to be interpreted as saying that X and Y mutually imply each other given any slot-filler, rather than just as saying X=Y. But this is doable within the language, since all we need to do is quantify over the slot-filler-names.

This can be thought of as my way of interpreting Frege's concept of the "extension" of a sentence-with-hole. Rather than being a seperate entity, the extension is a "view" of the sentence: the sentence up-to-equivalence.