On 3/14/2023 11:38 AM, Ross Finlayson wrote:
> On Monday, March 13, 2023
> at 10:09:55 PM UTC-7, Ross Finlayson wrote:
>> On Monday, March 13, 2023
>> at 4:44:43 PM UTC-7, Jim Burns wrote:
>>> We know that
>>> infinity is not
>>> a reallyreallyreallyreallyreallyreally large
>>> natural number.
>>
>> Yeah, but isn't "infinity", according to numbers,
>> very large?
Infinity is what's left over, once we've described
finity.
There are a zillion different things to be described
other than finity. Finity gets good press, though,
because so much of _what we care about_ is finite.
For example, us.
Here's what I think you (RF) mean:
|
| There are the large finites, and then ω
| There are the really large finites, and then ω
| There are the reallyreally large finites,
| and then ω
| There are the reallyreallyreally large finites,
| and then ω
| There are the reallyreallyreallyreally large
| finites, and then ω
| There are the reallyreallyreallyreallyreally large
| finites, and then ω
| There are the reallyreallyreallyreallyreallyreally
| large finites, and then ω
|
| Is ω different from
| a reallyreallyreallyreallyreallyreallyreally
| large finite?
Infinity is what's left over, once we've described
finity.
Describe finity: the finite ordinals.
| It is a finite ordinal iff,
| for each split between it and 0
| some i is last-before the split
| and some j is first-after the split.
Notation:
If j is first-after i, write j = i+1
We've formalized the notion of being able
to get there from here. For each split of
ordinals "on the way" to a finite ordinal,
there is a step across that split:
ordinals before and after next to each other.
Finity described.
The description is true of each finite ordinal.
That's not something finite-us can "check",
even in principle. Nonetheless, we know it
because we know what we mean by "finite ordinal".
Compare to:
We don't (can't, needn't) check that each triangle
has three corners.
>> Yeah, but isn't "infinity", according to numbers,
>> very large?
We can extend our discussion beyond finite ordinals
to _not-necessarily-finite_ ordinals by loosening
our restriction on (AKA our definition of) what we are
talking about.
| It is a not-necessarily-finite ordinal iff,
| for each split between it and 0
| ⬚⬚⬚ ⬚ ⬚ ⬚⬚⬚⬚⬚⬚ ⬚⬚⬚ ⬚⬚⬚
| ⬚⬚⬚ some j is first-after the split.
Clearly, finite ordinals also satisfy the definition
of not-necessarily-finite ordinals.
In addition, for each _finite_ ordinal,
for each split between it and 0
some i is last-before the split.
All the last-befores ==
we can get there from here.
Infinity is what's left over, once we've described
finity.
If there is some λ left over,
after describing the finites,
then there is a split between the finites,
(we can get there from here)
and the others
(we can't, not even in principle)
There is a first-after of that split.
We name the first-after ω
The difference between ω and the large,
really large, reallyreally large,
reallyreallyreally large,
reallyreallyreallyreally large,
reallyreallyreallyreallyreally large,
reallyreallyreallyreallyreallyreally large, and
reallyreallyreallyreallyreallyreallyreally large
numbers is that you _can_ get there from here (0)
for all of those, but not for ω
This is the explanation for the various
results that contradict what we would get for
a reallyreallyreallyreallyreallyreally large
natural number.
Infinity is not
a reallyreallyreallyreallyreallyreally large
natural number.
It is something else, with different properties.