[math] Accounting for Numbers

Max Shron max at shron.net
Wed Nov 14 11:35:19 CST 2012


On Wed, Nov 14, 2012 at 11:47 AM, Andrew Dudzik <adudzik at gmail.com> wrote:

> Weirdly, Landsburg seems to totally overlook the fact that most of us have
> been trained, for decades, to think of numbers in a particular way.  It may
> be parsimonious to think of numbers as existing "out there", but it's
> probably even more parsimonious to think of them the way we think of
> hopscotch, namely as some mechanical process common between people,
> possessing fad-like qualities, that we may not remember learning.
>
> Surely, Lansburg would smugly object that people have thought about
> numbers for longer than there have been public school systems, and this is
> true enough.  But David Graeber has pointed out (see his book on debt) that
> not only mathematics, but philosophy as well, apparently emerged (in Rome,
> India, and China, around the same time) *in response* to coinage.  It
> seems that nobody particularly cared that 2+2=4 until there were men with
> sharp things willing to stab you if you didn't give them exactly 4
> somethings.  In other words, the money system itself is an indoctrination
> into a particular way of thinking about numbers and logic.
>
> See, for example, the Pirahã people, who, contrary to Lansburg's assertion
> that the natural numbers are "directly accessible", have no number system
> at all, and appear to have lost the ability to acquire one by adulthood.
>  They have sought outside help in understanding mathematics only because of
> concerns about being ripped off by their neighbors in trade.
>

There is some neat research on how our innate number systems are more
logarithmic than linear by intuition (I believe this gets discussed in
*Thinking
Fast and Slow*). We're good at estimating orders of magnitude but need to
learn how to add beyond very small numbers. You can see this when you ask
people to compare numbers that are outside their experience, as when
politicians exploit the confusability of billions and millions or when you
try to talk about evolutionary time scales with most people.

Then again, I've always been a bit skeptical of the debate about the
> ontology of mathematics, which strikes me as rather like analyzing the
> political opinions of a corpse.  The subtext here is: We already agree
> about the details of mathematics, we just need to discuss what tie it
> should wear when we take it to meet Mother.  But there are far deeper
> potential disagreements within mathematics itself, many of which would
> spring to life if we loosened the stranglehold that the hegemonic
> university system holds on career and research.
>

I'd love to hear some more thoughts on this.



>
> On Wed, Nov 14, 2012 at 6:02 AM, Mike <michaelbish at gmail.com> wrote:
>
>>
>>
>> Sent to you by Mike via Google Reader:
>>
>>
>> Accounting for Numbers<http://www.thebigquestions.com/2012/11/14/accounting-for-numbers/>
>>  via Steven Landsburg | The Big Questions: Tackling the Problems of
>> Philosophy with Ideas from Mathematics, Economics, and Physics<http://www.thebigquestions.com>by Steve Landsburg on 11/13/12
>>
>> Over at Less Wrong <http://lesswrong.com/lw/f4e/logical_pinpointing/>,
>> the estimable Eliezer Yudkowsky<http://en.wikipedia.org/wiki/Eliezer_yudkowsky>attempts to account for the meaning of statements in arithmetic and the
>> ontological status of numbers. I started to post a comment, but it got long
>> enough that I’ve turned my comment into a blog post. I’ve tried to
>> summarize my understanding of Yudkowsky’s position along the way, but of
>> course it’s possible I’ve gotten something wrong.
>>
>> It’s worth noting that every single point below is something I’ve blogged
>> about before. At the moment I’m too lazy to insert links to all those
>> earlier blog posts, but I might come back and put the links in later. In
>> any event, I think this post stands alone. Because it got long, I’ve
>> inserted section numbers for the convenience of commenters who might want
>> to refer to particular passages.
>>
>> 1. Yudkowsky poses, in essence, the following question:
>>
>> *Main Question, My Version:* *In what sense is the sentence “two plus
>> two equals four” meaningful and/or true?*
>>
>> Yudkowsky phrases the question a little differently. What he actually
>> asks is:
>>
>> *Main Question, Original Version:* *In what sense is the sentence “2 + 2
>> = 4″ meaningful and/or true?”*
>>
>> This, I think, threatens to confuse the issue. It’s important to
>> distinguish between the numeral “2″, which is a formal symbol designed to
>> be manipulated according to formal rules, and the noun “two”, which appears
>> to name something, namely a particular number. Because Yudkowsky is asking
>> about meaning and truth, I presume it is the noun, and not the symbol, that
>> he intends to mention. So I’ll stick with my version, and translate his
>> remarks accordingly.
>>
>> 2. Yudkowsky provisionally offers the following answer:
>>
>> *First Provisional Answer:* *The sentence “two plus two equals four”
>> means that the expression “2 + 2 = 4″ is a valid inference from the axioms
>> of Peano arithmetic.*
>>
>> He then provisionally rejects this provisional answer on the grounds
>> (with which I wholeheartedly agree) that “figuring out facts about the
>> natural numbers doesn’t feel like the operation of making up assumptions
>> and then deducing conclusions from them.” He goes on to say: “It feels like
>> the numbers are just *out* there, and the only point of making up the
>> axioms of Peano Arithmetic was to allow mathematicians to talk about them.”
>>
>> He’s certainly right that it feels — to me, and, I am sure to almost
>> everyone who has ever thought much about arithmetic — like the numbers are
>> just “out there”. On the other hand, I’d quibble with Yudkowsky’s
>> assessment of the point of Peano arithmetic. The point isn’t to “allow
>> mathematicians to talk” about numbers; mathematicians from Pythagoras
>> through Dedekind had absolutely no problem talking about numbers in the
>> absence of the Peano axioms. Instead, the point of the Peano axioms was to
>> *model* what mathematicians do when they’re talking about numbers. Like
>> all good models, the Peano axioms are a simplification that captures
>> important aspects of reality without attempting to reproduce reality in
>> detail.
>>
>> 3. To reach a closer understanding of what numbers *are*, Yudkowsky
>> imagines trying to explain them to a logician with a full grasp of logic
>> but no grasp of numbers. Here I think Yudkowsky has fooled himself into
>> imagining an impossibility. If you grasp logic, you grasp the idea of a
>> proof. If you grasp the idea of a proof, you grasp the idea of a sequence
>> of logical steps. If you grasp the idea of a sequence of logical steps, you
>> grasp the idea of a sequence. If you grasp the idea of a sequence, you
>> already know a lot about numbers. This is one reason why I believe that any
>> attempt to account for numbers via logic must ultimately be circular.
>>
>> 4. Be that as it may, Yudkowsky goes on to try to explain to his
>> fictional interlocutor what numbers are. He begins by essentially stating
>> the first order Peano axioms: 0 is a number, every number has a successor,
>> no two numbers have the same successor, and so forth. Eventually, he
>> realizes that this approach isn’t taking him quite where he wants to go and
>> makes a bit of a course correction (as we’ll see below). But I think more
>> than a course correction is called for; he’s gone off in entirely the wrong
>> direction. He’s listing the *properties* of numbers, but not even *trying
>> * to explain what they *are*. If I were explaining numbers to a naif,
>> I’d probably start with something like Bertrand Russell’s account of
>> numbers: We say that two sets of objects are “equinumerous” if they can be
>> placed in one-one correspondence with each other; a “number” is that which
>> all sets equinumerous to a given set have in common. Whether or not that
>> works in detail, it’s at least an attempt at a definition, as opposed to a
>> mere list of properties.
>>
>> 5. Yudkowsky, in his fictional conversation with his fictional logician,
>> eventually comes to realize that neither the first order Peano axioms nor
>> any other first-order system can uniquely characterize the natural numbers.
>> This is a consequence of Godel’s Incompleteness Theorem<http://www.thebigquestions.com/2009/11/25/godel-in-a-nutshell/>,
>> or even more fundamentally of the Lowenheim-Skolem Theorem. What it means
>> is that no matter what axioms you start with, there are going to be
>> multiple systems that satisfy those axioms; the natural numbers are only
>> one of those systems, so your axioms cannot collectively specify the
>> natural numbers.
>>
>> 6. Yudkowsky solves his problem by passing to second order Peano
>> arithmetic — “second order” meaning that, in addition to using variables to
>> represent numbers, you can also use variables to to represent *sets* of
>> numbers. He correctly notes that second order Peano arithmetic has a unique
>> model. (I am using the word “model” here in the technical sense of logic,
>> not in the informal social-sciencey sort of way that I used it in point 2
>> above.) This means that sure enough, there is one and only one system that
>> satisfies all the axioms of second-order arithmetic. And he concludes that:
>>
>> *Y’s conclusion*: *That’s why the mathematical study of numbers is
>> equivalent to the logical study of which conclusions follow inevitably from
>> the number-axioms.*
>>
>> But this is disastrously wrong for at least two reasons, each of which
>> deserves its own numbered point.
>>
>> 7. Yudkowsky leaps from “the natural numbers can be precisely specified
>> by second order logic” to “the .. study of numbers is equivalent to the
>> logical study of which conclusions follow inevitably from the
>> number-axioms”. This is wrong, wrong, wrong, because *second order logic
>> is not logic*<http://www.thebigquestions.com/2010/07/27/first-things-and-second-things/>.
>> Indeed, the whole point of logic is that it is a *mechanical* system for
>> deriving inferences from assumptions, based on the *forms* of sentences
>> without any reference to their *meanings*. (Thus if we assume that all
>> bachelors are unmarried and that Walter is a bachelor, we can infer that
>> Walter is unmarried, without having to know anything at all about who
>> walter is, or what the words “bachelor” and ‘unmarried” mean.) That’s why
>> you’re not allowed to set up an axiom system in which all the true theorems
>> of arithmetic are taken as axioms — there is no mechanical procedure for
>> determining whether a given statement is or is not a true theorem of
>> arithmetic (see Tarski’s theorem on the undefinability of truth) and
>> therefore no mechanical procedure for determining what is or is not an
>> axiom in that system. In second-order Peano arithemetic, we have an
>> analogous problem: The *axioms* can be identified mechanically, but the *rules
>> of inference* can’t. A properly programmed computer can examine a
>> first-order proof and tell you if it’s valid or not; that is, it can tell
>> you whether each step does in fact follow logically from some of the
>> previous steps. But no computer can do the same for second-order proofs.
>>
>> So the study of second-order consequences is not logic at all; to tease
>> out all the second-order consequences of your second-order axioms, you need
>> to confront not just the *forms* of sentences but their *meanings*. In
>> other words, *you have to understand meanings before you can carry out
>> the operation of inference*. But Yudkowsky is trying to *derive* meaning
>> from the operation of inference, which won’t work because in second-order
>> logic, meaning comes first.
>>
>> 8. Even putting all that aside, Yudkowsky is relying on a *theorem* when
>> he says that second-order Peano arithmetic has a unique model. That theorem
>> requires a substantial dose of set theory. So in order to avoid taking
>> numbers as primitive objects, he’s effectively resorted to taking *sets*as primitive objects. But why is it any more satisfying to take set theory
>> as “given” than to take numbers as “given”? Indeed, the formal study of
>> numbers precedes the formal study of sets by millennia, which suggests that
>> numbers are a more natural starting point than sets are. Whether or not you
>> buy that argument, it’s important to recognize that Yudkowsky has “solved”
>> the problem of accounting for numbers only by reducing it to the problem of
>> accounting for sets — except that he hasn’t even done that, because his
>> reduction relies on pretending that second order logic is logic.
>>
>> 9. All of which leaves us with the problem of accounting for numbers, and
>> for the meaning of statements like “two plus two equals four”. To me, by
>> far the most satisfying solution is a full-fledged Platonic
>> acknowledgement <http://www.thebigquestions.com/2010/01/13/real-numbers/>that numbers are indeed just “out there” and that they are directly
>> accessible to our intuitions. I embrace this view for (at least) three
>> reasons: A. After a lifetime of thinking about numbers, it feels right to
>> me. B. Pretty much every one else who spends his/her life thinking about
>> numbers has come to the same conclusion. C. It seems enormouosly more
>> plausible to me that numbes are “just out there” than that physical objects
>> are “just out there”, partly because there is in fact a unique system of
>> (standard) natural numbers, whereas the properties of the physical universe
>> appear to be far more contingent and therefore unnecessary. I’ve given an
>> account in *The Big Questions*<http://www.amazon.com/Big-Questions-Philosophy-Mathematics-Economics/dp/143914821X/ref=nosim/?tag=moseissase-20>of how the existence of numbers can account for the existence of the
>> physical universe; I think it would be very difficult to go in the opposite
>> direction (though I’ve seen some pretty good attempts). Therefore, accepting
>> numbers as primary<http://www.thebigquestions.com/2010/12/02/first-things/>and accounting for the universe as a necessary consequence of numbers seems
>> to me to be the ontologically parsimonious<http://www.thebigquestions.com/2012/09/18/wwct-what-would-copernicus-think/>thing to do, and I like parsimony.
>>
>> 10. Needless to say, point 9 is not a proof. But I know of no alternative
>> story that strikes me as even remotely plausible. Moreover, the alternative
>> stories all seem to go wrong in pretty much the same ways; for example,
>> every single point above is one I’ve blogged<http://www.thebigquestions.com/2010/09/01/basic-arithmetic-on-what-there-is/>
>> about<http://www.thebigquestions.com/2010/08/18/basic-arithmetic-part-ii/>
>> before<http://www.thebigquestions.com/2010/08/19/basic-arithmetic-part-iii-the-map-is-not-the-territory/>in other contexts, but here they are, all being relevant again.
>>
>> Share/Save<http://www.addtoany.com/share_save#url=http%3A%2F%2Fwww.thebigquestions.com%2F2012%2F11%2F14%2Faccounting-for-numbers%2F&title=Accounting%20for%20Numbers>
>>
>>
>>
>> Things you can do from here:
>>
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