A_Seagull wrote: But why this concern with 'completeness'?
I should perhaps have been a bit clearer in my definitions. By completeness here I am referring to the completeness of the Reals. That is, using Dedekind Cuts we can easily prove that there are no gaps in the real numbers. There of course remains the complex numbers which are required to solve for the roots of certain polynomials. I meant completeness only in this fashion. I think that the continuity of the Reals is a fairly important property to establish. Though one can argue that the definition of a Real Number via a Dedekind cut is really "no better off" than merely accepting the existence of the Reals axiomatically, in my opinion the ability to prove certain qualities (such as completeness) using an abstract logical framework provides a more satisfying foundation. I can only speak from personal experience, but when I first encountered Dedekind Cuts in Real Analysis I felt I had achieved a much deeper understanding of the concept of "number" than I had previously held. I don't think mathematics
requires the Dedekind Cut definition, but I do think it is enriched by it.
A_Seagull wrote: If I have a real number - take 1.5 for example, that defines it - at least within the general framework of mathematics. I do not need any further definition.
Well that isn't precisely true. When you use the symbol 1.5 you have an idea in your mind of what that entity is. You may think of one and one half for instance, which is built upon the dual definitions of "one" and the concept of "half." The question of course is what is "one" or "1?"We may choose to
define the object 1 as the base unit object and to
define addition such that 1+1=2 where 2 is
defined as the object 1+1. This agrees well with our experience in real world enumeration because we can easily refer each integer back to a group of objects which it enumerates and verify our arithmetic with physical stand ins, it does however, (in my humble opinion) lack a degree of 'elegance.' We also become a bit befuddled in attempting to tie the irrationals, and certainly the transcendental numbers, down via our experience with real world objects.
By using an abstract notion of number not tied to our experience of enumeration (I hope I have convinced you that we must have some concept of what 1.5 is which extends beyond the printing of symbols to paper) we are able to view addition, subtraction, multiplication, enumeration, etc. as a logical process easily defined and applied to independent abstract entities rather than having to define a number in terms of its relation to a base unit of enumeration. Thus number 1.5 is indeed "one and one half" but this is a property which evolves out of the definition of number as a cut in the continuum.
Here I must admit I feel I am not fully conveying the essence of the idea which is involved. Perhaps the best that I can say is that, personally, I view that Dedekind cut definition as conveying a much more satisfying sense of what a number is. That is, that a number is not an enumeration of a group of objects, but rather a sort of sharp point in the continuum. I think this view of number lends itself very well to Real Analysis and especially to the concept of probability where we can view numbers as stand in tokens for statistical events. For instance I think the value of a probabilistic retrieval of an integer from the line at a given draw is more intuitively understood when viewing number from this perspective.
A_Seagull wrote: Bertrand Russell's 2 volume 'proof' of '1+1=2' seems to me to be a total absurdity.
I more or less agree. While his motives were sound I doubt too much good could have come from such a reduction in the best of circumstances. Thanks to Godel we now know that the ultimate good sought after (namely the ability to create a writeable system in which all of mathematics could be drawn up algorithmically) is impossible. Had this not been the case the effort may have produced a system for radically advancing mathematics. Since the overall program was proven an impossibility however I don't think anything useful can ever be obtained from the effort.
A_Seagull wrote: My understanding is that Godels work followed from Hilbert's suggestion that mathematics might be finitely describable, consistent and complete. So, Godel proved that this could not be so.
Yes, Godels theorems are often seen as a negative solution to Hilbert's second problem.
A_Seagull wrote: Clearly the axioms should be finitely describable and if inconsistency is found then this is not a 'good' thing. But there is no necessity for completeness of the system. The theorems generated from the axioms can happily continue wihout check or covering every possibility.
The problem is of course in the finding of the inconsistency. Godel's second theorem demonstrates that no such system can be proven consistent. Also while it is true that mathematics does not need to prove every possible theorem to be a useful and worthwhile endeavor it is somewhat unsettling that there must exist theorems in any "theory" (using Godel's language) of arithmetic which are forever beyond the bounds of proof. Would we be satisfied with a mathematical formalism which forever precluded the answering of the Riemann Hypothesis? I doubt it.
Godel's work speaks to our inability to "explain mathematics to a computer" as it were. Some have interpreted this as having some bearing on the ability of organic humans to transcend the capability of digital processes and used it as a "proof" that we are more than biological machines. I think this is overstepping the theorems bounds. It does however engender certain questions regarding the foundations of mathematics and our ability to comprehend the absolute roots of this logical system.
As another passing note: Have you ever heard of or read the book
Godel, Escher, Bach. An Eternal Golden Braid ? It has been a while since I first picked it up and I should go back and read it again now that I actually know a bit more about what the author is talking about. If you're interested in these sorts of questions you may find it an enjoyable read.