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Alfred Tarski: Life and Logic
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Mathematical logic: index, glossary, FAQ

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What is Mathematics:
Gödel's Theorem and Around

Hyper-textbook for students,
by Karlis Podnieks, Associate Professor
University of Latvia
Institute of Mathematics and Computer Science

An extended translation of the 2nd edition of my book " Around Goedel's theorem" published in 1992 in Russian.

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My Main Theses

I define mathematical theories as stable self-contained systems of reasoning, and formal theories - as mathematical models of such systems. Working with stable self-contained models mathematicians have learned to draw a maximum of conclusions from a minimum of premises. This is why mathematical modeling is so efficient.

For me, Goedel's results are the crucial evidence that stable self-contained systems of reasoning cannot be perfect (just because they are stable and self-contained). Such systems are either very restricted in power (i.e. they cannot express the notion of natural numbers with induction principle), or they are powerful enough, yet then they lead inevitably either to contradictions, or to undecidable propositions.

For humans, Platonist thinking is the best way of working with stable self-contained systems. Thus, a correct philosophical position of a mathematician should be: a) Platonism - on working days - when I'm doing mathematics (otherwise, my "doing" will be inefficient), b) Advanced Formalism - on weekends - when I'm thinking "about" mathematics (otherwise, I will end up in mysticism). (The initial version of this aphorism is due to Reuben Hersh).

Next step

The idea that stable self-contained system of basic principles is the distinctive feature of mathematical theories, can be regarded only as the first step in discovering the nature of mathematics. Without the next step, we would end up by representing mathematics as an unordered heap of mathematical theories!

In fact, mathematics is a complicated system of interrelated theories each representing some significant mathematical structure (natural numbers, real numbers, sets, groups, fields, algebras, all kinds of spaces, graphs, categories, computability, all kinds of logic, etc.).

Thus, we should think of mathematics as a "two-dimensional" activity. Most of a mathematician's working time is spent along the first dimension (working in a fixed mathematical theory, on a fixed mathematical structure), but, sometimes, he/she needs also moving along the second dimension (changing his/her theories/structures or, inventing new ones).

Do we need more than this, to understand the nature of mathematics?

What is Mathematics?

Four definitions of mathematics provably equivalent to the above one (see Section 1.2):

Mathematics is the part of science you could continue to do if you woke up tomorrow and discovered the universe was gone.
I do not know the author of this elegant definition put on the web by Dave Rusin.

The human mind has first to construct forms, independently, before we can find them in things.
Albert Einstein, see Quotations by Albert Einstein

In mathematics you don't understand things. You just get used to them.
John von Neumann, see Quotations by John von Neumann

Mathematicians are mad tailors: they are making "all the possible clothes" hoping to make also something suitable for dressing...
Stanislaw Lem, "Summa Technologiae" (sorry - my own English translation, the initial version of this aphorism may be due to David van Dantzig, see Quotations by David van Dantzig)

Goedel and Einstein

Why is Einstein much more popular than Goedel?
See Kurt Godel in Blue Hill, by Peter Suber

Wir muessen wissen -- wir werden wissen!
David Hilbert's Radio Broadcast, Koenigsberg, 8 September 1930
(audio record published by James T.Smith, and translations in 7 languages published by Laurent Siebenmann).

"Hilbert and Goedel never discussed it, they never spoke to each other. ... They were both at a meeting in Koenigsberg in September 1930. On September 7th Goedel off-handedly announced his epic results during a round-table discussion. Only von Neumann immediately grasped their significance..." (G.J.Chaitin' s lecture, 1998, Buenos Aires)

See portraits of these brilliant people in the MacTutor History of Mathematics archive
at the University of St Andrews:

Plato     Georg Cantor    David Hilbert    Ernst Zermelo    Kurt Goedel    Paul Cohen