[FOM] Convincing Edward Nelson that PA is consistent
Arnon Avron
aa at tau.ac.il
Wed Jun 20 16:33:21 EDT 2018
Tim,
In your posting you refer to two types of people:
the skeptical formalists (who are
somehow more reasonable than the strict formalists)
and the ultrafinitists. I understand from your message
that Nelson was both. But *should* an ultrafinitist
be a (skeptical) formalist? Conversely: *Should*
a skeptical (or even strict) formalist be an ultrafinitist?
If so - why? If not - how would you convince
a very skeptical formalist who is not an ultrafinitist
that PA is consistent?
Arnon
On Mon, Jun 18, 2018 at 11:56:39PM -0400, Timothy Y. Chow wrote:
> Sadly, of course, Nelson passed away a few years ago so there is no
> way we can literally convince him that PA is consistent. However,
> in the process of writing an expository article about the
> consistency of PA, I thought of a way that "normal" people could
> potentially make some progress on the project that Nelson was
> engaged in---though of course in the direction of showing that PA is
> consistent rather than inconsistent.
>
> Since Nelson's views were unusual, I have to give some philosophical
> preamble. I would like to introduce the term "strict formalist." A
> strict formalist is able to *follow* prescribed syntactic rules and
> can for example verify that a given mathematical proof (of feasible
> size) follows from given axioms, but takes very seriously the claim
> that the symbols and strings do not *mean* anything. In particular,
> the strings cannot refer to syntactic objects, and an expression
> such as "Con(PA)" cannot mean anything, let alone mean something
> about what happens if you manipulate the axioms of PA according to
> the rules of first-order logic. This is not just because "Con(PA)"
> is usually formulated as an arithmetical statement; even if we use
> something like Quine's protosyntax that is supposed to directly
> formalize syntactic reasoning, the strict formalist refuses to
> *interpret* protosyntax as "saying" anything about syntax.
>
> It's a pretty good approximation to think of a strict formalist as a
> computerized proof assistant. To drive home the point that strict
> formalists only *follow* rules and don't *reason* about them,
> imagine starting with the symbol "0" and then repeatedly applying
> the rule "prepend an S." The results, of course, will look like
> this: S0, SS0, SSS0, SSSS0, .... You and I look at this and can
> reason that at no point will we obtain a string that contains (say)
> a "+" sign in it. However, this observation is a piece of
> *mathematical reasoning* and is unavailable to the strict formalist.
> The strict formalist can verify on a case-by-case basis that "S0 has
> no + in it," "SS0 has no + in it," etc., but that is all.
>
> I introduce the concept of a strict formalist as a "baseline." The
> strict formalist is able to accomplish the main task that formalists
> are eager to affirm, namely recognizing syntactic objects,
> manipulating them according to certain rules, and verifying that the
> rules have been correctly applied, but is an absolute mathematical
> dullard. Implicitly, a strict formalist could verify that PA is
> inconsistent by checking the correctness of a PA proof of 0=S0, but
> would not be able to perk up and say, "Hey, look at that---PA is
> inconsistent!" It is therefore impossible to present an argument to
> a strict formalist that will get the strict formalist to affirm (in
> English) that "this system is *inconsistent*" let alone the more
> subtle claim that "this system is *consistent*" for the trivial
> reason that strict formalists are not in the business of affirming
> mathematical statements in English.
>
> In practice, I don't think any human being is a strict formalist.
> In Nelson's paper "Taking Formalism Seriously," he doesn't take
> formalism as seriously as a strict formalist does. At the end, he
> talks somewhat vaguely about proving that some weak system is
> consistent in a way that will "convince the most skeptical of
> formalists." Of course, a strict formalist is even more
> intransigent than what Nelson envisages as "the most skeptical of
> formalists," but the point is that the concept of a strict formalist
> lets us see that Nelson is slipping in some extra assumptions.
> Namely, Nelson is tacitly allowing the possibility that *some kinds
> of mathematical arguments can provide us with secure knowledge about
> syntactic objects*. The symbols used to record these mathematical
> arguments are not just meaningless symbols, to be manipulated
> according to certain rules; they *tell us something* about the
> behavior of syntactic entities.
>
> To my knowledge, Nelson was never explicit about exactly what
> mathematical principles "the most skeptical of formalists" would
> accept. I would presume that Nelson would allow us to conclude "PA
> is inconsistent" if we were presented with an explicit proof of a
> contradiction from PA. I would also presume that Nelson would allow
> us to conclude that if all we do is repeatedly prepend "S" to "0"
> then we will never get a "+"---although even here, I think we have
> to be careful what "never" means, because Nelson is what I would
> call an ultrafinitist. The sequence S0, SS0, SSS0, ... gradually
> fades off into meaninglessness because the strings get infeasibly
> large. Characterizing this "fading off" in a way that is
> mathematically analyzable is notoriously difficult and probably
> can't be done in a fully satisfactory manner; however, one can try
> to work around it by setting some upper and lower bounds on the
> fading off point (maybe 2^1000 and 2^100 respectively, using
> conventional mathematical notation).
>
> Now here is an important step: I maintain that the most plausible
> way to interpret "PA is consistent" in this context is "There is no
> *feasible* PA proof of 0=S0." Again, exactly what this means is
> tricky, but the conventional mathematical claim that "There is no PA
> proof of 0=S0 shorter than 2^1000 symbols" should imply it.
>
> We're now in a position to pose a question that makes sense to a
> conventional mathematician: What are the weakest mathematical
> principles that allow us to deduce "There is no PA proof of 0=S0
> shorter than 2^1000 symbols"?
>
> What makes this particularly tricky is that "the most skeptical of
> formalists" isn't going to be happy interpreting "2^1000" the way a
> conventional mathematician does. So we have to be careful when we
> deal with exponential notation, and perhaps even multiplicative
> notation. At the same time, if we are to have any hope of making
> progress, we have to allow *some* principles that allow us to prove
> theorems of length n that "say something about" syntactic entities
> of length N when N >> n. Otherwise we can't approach "PA is
> consistent" other than by direct exhaustion. I'm not sure exactly
> how to proceed at this point, but if this issue can be addressed,
> then I can imagine that in principle, "the strictest of formalists"
> could become convinced that "PA is consistent," by carrying out
> something like the following:
>
> 1. Do a computation with 1000 steps to arrive at a theorem that says
> that there is no PA proof of 0=S0 of length 1000000.
>
> 2. Do a computation with 1000000 steps to arrive at a theorem that
> says that there is no PA proof of 0=S0 of length 1000000000000.
>
> 3. Etc.
>
> Here I've arbitrarily picked an "expansion factor" of n -> n^2.
> That means that by getting up to the square root of the feasibility
> bound, the "most skeptical of formalists" can become convinced that
> there is no feasible contradiction. I can imagine developing some
> kind of sliding scale, where progressively stronger mathematical
> principles give you bigger expansion factors. Very skeptical
> formalists might have to get up to the square root of the
> feasibility bound to be convinced that PA is consistent, while less
> skeptical formalists might only have to go up to polylog.
>
> Tim
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