On 5/27/2022 7:27 AM, David Petry wrote:
> First of all, anyone who claims that
> Cantor's set theory is in any way essential for
> the mathematics that is relevant to science and
> technology simply doesn't know what he's
> talking about.
I have a different claim.
Science-and-technologists choose to use
mathematics based on Cantor's set theory,
and,
as best we can tell, there is no reason for
them to choose differently.
> For thousands of years mathematicians accepted
> the idea that mathematics is closely related to science,
> and that since the notion of a completed (or "actual")
> infinity is not part of science, it is also not
> part of mathematics.
>
> The mathematicians did, however, accept the idea of
> a potential infinity.
The terms "actual infinity" and "potential infinity"
are going to cause problems in this discussion.
One poster (Can you guess who?) has taken them for
his own, and uses them in ways others do not.
> We can think of infinity as a destination that
> can never be reached. That is, no matter how far we go
> in the direction of infinity, there's still an infinite
> distance beyond that to go. Note that this is true in
> science, and hence it must be true in mathematics
> if mathematical reasoning is to be compatible with
> scientific reasoning.
There doesn't seem to be room for the real numbers
in this description of infinity. Speaking as
a sometime-dabbler in scientific reasoning,
I want my real numbers.
I want my description of a continuum.
Essential or not, describing things with a continuum
can be useful -- if in no other way, then useful
to my imagination. I choose to use real numbers,
if I am not prevented from doing so.
I think that what makes a continuum continuum-y
is that there are enough points in it that a
function which is _continuous at each point_
cannot have the leaps that we associate with
discontinuities.
Consider _only_ the positive rationals.
The function f
f(x) =
{ 0 for x^2 < 2
( 1 for x^2 > 2
is continuous at every point --
at every _rational_ point.
And yet, there is this leap at ...
well, we can't say where, really.
That seems to be the root of the problem.
So, we include sqrt(2) in our continuum.
f(sqrt(2)) must have a value, but, no matter what
it is, f(x) can't be continuous at sqrt(2).
This is the correct answer.
When we consider continuous functions,
we want to exclude f(x).
Generally,
when we partition the continuum into LEFT and
RIGHT, each point in LEFT to the left of each
point in RIGHT,
we want a point between LEFT and RIGHT at which
functions can be continuous, thus restricting
"continuous functions" to _what we mean_ by
"continuous functions".
In other words,
being continuum-y requires Dedekind completeness.
However,
Dedekind completeness requires us to lay aside
the notion of infinity as endless sequence.
No endless sequence of points can contain
all of the points of the continuum.
From an endless sequence of points, we can partition
the continuum into LEFT, RIGHT, and BETWEEN,
with no sequence-point in BETWEEN.
If there are no points in BETWEEN, we can define
some g(x) continuous everywhere in LEFT and in RIGHT
but which makes some leap in going from LEFT to
RIGHT.
We don't want that, our continuum wouldn't be
continuum-y. But points in BETWEEN aren't
in the endless sequence. Therefore, the endless
sequence doesn't contain all points.
And that's true of any endless sequence of points.
This is what I mean by saying
there isn't enough room in the endless-sequence-style
infinity for real numbers.
A reminder: I want continuum-y real numbers.
> The claim behind the notion of an actual infinity is
> that even though we cannot in any meaningful sense
> actually reach infinity, we can talk about infinity
> as if we could reach it.
I am going to excuse myself from discussion of what
should or should not be called "actual infinity".
It seems to me that ________ infinity can be
discussed by describing _one of_ some collection
which happens to be infinite, and then using
only truth-preserving inferences to proceed from
that description to further claims about that
collection.
For example,
we can describe _one of_ the collection of cuts
LEFT and RIGHT of the continuum as having a
point-between. That makes the continuum
sufficiently continuum-y for our purposes.
And we go on from there, truth-preserving-ly.
> What I have been claiming for over 32 years now,
> is that there have been some major advances in
> our understanding of scientific reasoning over
> the past century, and that these advances in our
> understanding of scientific reasoning could be
> formalized and integrated into mathematics at
> the foundational level.
32 years is a long time.
It seems to me that foundations are intended
to support whatever we choose to build on them.
Mixing metaphors, a good foundation is a Swiss Army
knife. We won't necessarily find a use for the
spork. That's not a reason to remove the spork.
It sounds to me as though you want to remove
all "unnecessary" tools from our foundations.
Like removing the spork from a Swiss Army knife.
I don't see any good argument for doing so.
> And sometimes they go so far as to claim that
> what I want to do is evil,
I don't think formalizing falsification in
mathematics is evil. I question how useful it
would be. The philosophy of science has moved on
from falsification. But no one needs to pay any
attention to my questions.
I think that Lysenkoism is at least anti-truth
and could be very evil indeed, depending upon
how its regulations are enforced.
At various times, you have sounded as though
you were advocating some form of Lysenkoism.
> and that I must be motivated by a desire to deprive
> the mathematicians of their intellectual freedom.
> And what they accuse me of doing is what they are doing.
32 years is a long time.
What if you had spent 32 years working on whatever
you wanted to work on, such as on formalizing
falsification, and then whoever was interested in
your work joined in?
Is there something wrong with the picture I paint?