Math Forum Discussions

Alan Smaill

Posts: 717
Registered: 1/29/05

Re: Matheology § 203
Posted: Feb 1, 2013 8:56 PM

Plain Text

WM <mueckenh@rz.fh-augsburg.de> writes:

> On 1 Feb., 16:38, Alan Smaill <sma...@SPAMinf.ed.ac.uk> wrote:
>> William Hughes <wpihug...@gmail.com> writes:
>> > Let P(n) be
>> > 0.111... is not the nth line
>> > of
>>
>> > 0.1000...
>> > 0.11000...
>> > 0.111000...
>> > ...
>>
>> > Clearly for every natural number n
>> > P(n) is true.
>>
>> > This means there is no natural
>> > number m for which P(m) is true.
>>
>> > It is not simply that we cannot find m,
>> > we know that m does not exist.
>>
>> Futhermore WM accepts, for example, that
>> for every natural number n, 2 * n is even.
>>
>> Doesn't he?
>
> Of course.

Nothing is "of course" when the Prophet speaks.

>> But when asked how it is possible to know such a thing,
>> he falls strangely silent.
>
> For that theorem you need not know (actually) all natural numbers.
> Induction is sufficient that holdes for (potentially) all natural
> numbers, i.e., up to every natural number. There is no impredicative
> definition involved.

More matheology from the Prophet!

http://logic.harvard.edu/EFI_Parsons_StrictPredicativity.pdf

"The logicist reduction of the concept of natural number met a
difficulty on this point, since the definition of ?natural number?
already given in the work of Frege and Dedekind is impredicative. More
recently, it has been argued by Michael Dummett, the author, and Edward
Nelson that more informal explanations of the concept of natural number
are impredicative as well. That has the consequence that impredicativity
is more pervasive in mathematics, and appears at lower levels, than the
earlier debates about the issue generally presupposed."

So, how on earth do you know that induction is a correct
principle over the natural numbers?

You only ever have finitely many of them, so you can never know
what will happen when you look at a new one.

>
> Regards, WM

--
Alan Smaill