Okay. It sounds as though we are on the same page here.
Rupert has already offered to prove this to you, and I
don't want to steal any thunder from him.
However, I will offer a quick preview of the proof
(because I find it so beautiful).
Suppose we have the rationals, with the usual operations
on the rationals, '+', '-', '*', '<', those necessary
in order to be an ordered field, such as the rationals or
the reals or some other ordered field.
Suppose we have sets of rationals. In particular, suppose
we have the set D of all sets x of rationals with these four
properties:
Let x e D.
x is not empty
(Ea) a e x
x is not Q
(Ea) ~(a e Q)
x continues all the way down, without holes.
(Aa)(Ab) ( a e x & b < a ) -> b e x
x does not contain a maximum.
(Aa) a e x -> (Eb)( b e x & a < b )
Theorem:
The set of all such x is a complete ordered field.
(This is what Rupert offered to proved to you)
The a and b above here are rationals, and the '<' up there
is the '<' you would expect to use with the rationals.
Another order can be defined on these sets, which
can be proven to have the properties needed for the
order of an ordered field, plus one more property,
the _least upper bound property_ .
(This is a theorem, Ross, not an assertion of an axiom.
The assumption is that these sets in D exist.)
The newly defined order is set inclusion (as Rupert has
already mentioned). if x e D and y e D, then
x is less than y iff x c y and exactly one of
x c y , x = y , y c x is true, and so on.
Here's the beautiful part:
For every bounded, non-empty set A of elements of D
(the things we claim are real numbers because that
is how they behave) the _union of A_ is also in D
(satisfies those four conditions above) and is the
_least upper bound_ of A (bounds A and is less than
or equal to any other bound).
I haven't proven this for you, but it is provable, given no
more assumptions than we have already mentioned.
I think that you would agree that that this is emergent
from the properties of sets and rationals, and not
assumed.