On 1/12/2020 6:40 AM, Ganzhinterseher wrote:
> Am Samstag, 11. Januar 2020 22:15:49 UTC+1
> schrieb Jim Burns:
|> The rules can be written out as
|> 0/ ==> 1
|> 1/ ==> 2
|> 2/ ==> 3
|> 3/ ==> 4
|> 4/ ==> 5
|> 5/ ==> 6
|> 6/ ==> 7
|> 7/ ==> 8
|> 8/ ==> 9
|> 9/ ==> /0
|> [/ ==> [1
>> There is no infinity in this description of "increment.
>
> But set theory needs it.
First order logic ("set theory") is able to describe infinite
things through the use of a formal theory of "all" (variables
and quantifiers). This theory of "all" is the same theory used
in (WM-good) "All numbers have successors" as in (WM-bad)
"The intersection of all end segments is empty".
Essentially, skipping technical details, this is
_the whole theory_
-- If all the things are P, and b is a thing, then b is P.
-- If there is a counter-example to being P, then not all
the things are P.
-- ~( Q \/ Ax:P(x) ) contradicts Ax:( Q \/ P(x) )
-- "Not all children like chocolate" does not mean
"All children do not like chocolate".
This is where you can find your "completed infinity" in our
work, in the formalization of uses of "all" that do not
*need* to distinguish between finite and infinite.
If you're going to object, object to _this_
But, if you do, bring an argument, not more blather.
[...]
> "Reaching the infinite" is the request and concern of
> set theory. But that is not possible with definable
> numbers.
How do we describe natural numbers in a way that includes
all the natural numbers and only the natural numbers?
This is a different question from "Are we there yet?"
which seems to be the question you _want_ us to ask. Or,
maybe, it's the question you insist that we _are asking_
despite being told otherwise almost infinitely many times.
Given a description of all and only the natural numbers,
we can draw some conclusions from that description about
all and only the natural numbers. Saying clearly what we
mean by "all" is key to an argument like this. "Reaching
the infinite" is NOT key, is NOT even in the argument.
----
How do we describe a sequence such as
0, 1, 2, ... , 99, 100
in such a way that it is NOT just that one sequence,
but, instead, that it would generalize to
0, 1, 2, ... , 100, 101
and
0, 1, 2, ... , 101, 102
and
0, 1, 2, ... , 98, 99
and
0, 1, 2, ... , 97, 98
and
_all_ such sequences.
What is "such a sequence"? This would get us most of
the way to an answer to "What is a natural number?"
Consider the collection { 3,5,4,7,1,0,2,6 }.
(i)
We want everything in the collection to have
a unique successor and a unique predecessor,
except for the first, 0, with only a successor,
and the last, 7, with only a predecessor.
(ii)
We want no extraneous loops, like a --> b --> c --> a.
So, the _collection_ as a whole should not have
any smaller collection satisfying our requirements.
The difference, that is, the numbers in "our" collection but
not in the (forbidden) smaller collection would be
extraneous. No smaller such collection: no extraneous "numbers".
(iii)
We do not want such a sequence to either wander off
"to infinity" or wander in "from infinity". We don't want
..., -5, -4, -3, 3, 4, 5, ...
This satisfies our other requirements, but we don't
want to include that as a sequence "from" 3 "to" -3.
(?) 3, 4, 5, ... -5, -4, -3
So, the collection should not be infinite.
And what is "infinite"? For a sequence, each having
another after -- the opposite of having a last one.
(Or, from another point of view, each having another before
-- the opposite of having a first one.) Here, being able
to say "all", as in "all items have a successor", is essential
to saying what we mean.
These, (i), (ii), (iii), are at least _necessary_ properties
of sequences like 0, 1, 2, ... , 99, 100
I think that we can show that they are also sufficient
to define natural numbers, but that's for another day.
----
I can imagine someone asking why I'm going to all of this
trouble, making something complicated out of something so
simple as 0, 1, 2, ... , 99, 100
In my opinion, it's because we have a theory of of "all" --
quantifiers and variables. We paint a portrait of the
natural numbers with whatever we have at hand, and this
is what we have at hand. Some oddness and clunkiness is
to expected, I think. I would compare it to (literal)
portraits made with whatever the artist has at hand, like
https://www.buzzfeed.com/alanwhite/stunning-portaits-made-from-found-objects
>> Because _what we want to represent_ are these
>> literals 0,1,2,...,99,100,... we define the natural
>> numbers so that there are no "other" natural numbers:
>> no dark natural numbers.
>
> But then there is no actual infinity.
If anyone knows what you mean by "actual infinity",
it would be you. Thus, I'll take your word for that.
Fine. There is no "actual infinity".
Note that we can reason about _all_ the natural numbers
by reasoning from properties that we define _all_ the
natural numbers to have, by using our theory of "all"
(variables and quantifiers). It would follow from there
being no "actual infinity" that we can do this without
"actual infinity".
Does it matter?