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A Step by Step Guide to Gödel’s Incompleteness Proof
1: Part 1 of Gödel’s paper




Part 1 of Gödel’s paper

This part of Gödel’s paper is an introduction and is not intended to be completely rigorous, so it should not be read with such expectations, and should be read as expository material that lays out an overview of the paper. This guide is intended to be read alongside the English translation of Gödel’s original proof which can be viewed online at English translation of Gödel’s original proof or as a PDF file at English translation of Gödel’s original proof, PDF file.


In the paragraph beginning “The development of mathematics…”, Gödel sets out his claim that all formal systems are incomplete. Then, in the paragraph beginning “Before going into details… ”, Gödel sets out the basic ideas underlying his proof; the principal ideas are as follows:


The definition of a formal system

A formal system consists of a definite set of symbols. The formal system includes definitions that define which combinations of the symbols are valid formulas of the system. A proof (or proof-schema, as Gödel puts it) in a formal system is simply a series of formulas, beginning with one or more axioms, where each formula in the series follows from one of more previous formulas by the rules of inference of the system - where the last formula is the formula that is proved by the proof-schema.


Note: Gödel refers to a formal system PM - this refers to Russell’s system ‘Principia Mathematica’. (Footnote: Russell, Bertrand, and Alfred North Whitehead, “Principia Mathematica”; 3 Volumes, 1910, 1912, and 1913, Cambridge University Press.) The formal system P that Gödel actually uses in his proof includes typed classes in a similar fashion to that in ‘Principia Mathematica’ together with the Peano axioms. (Footnote: The Peano axioms were formulated by the Italian mathematician Giuseppe Peano. They constitute a formal definition of the fundamental properties of natural numbers. See for example, The Peano Axioms at Wolfram.)


The mapping of the formulas of a formal system to numbers

The symbols of the formal system are mapped to natural numbers, so that for every symbol of the formal system, there is a corresponding unique number. Using this mapping, then a formula of the formal system becomes mapped to a series of natural numbers, and a proof-schema becomes mapped to a finite series of series of natural numbers. And although Gödel does not state it here, in fact, his proof includes a method by which each such series of numbers is transformed into a single number, so that for each formula, and for each proof/proof-schema, there can be a unique corresponding natural number.


Mapping of relationships between formulas to relationships between numbers

Since relationships between formulas of the formal system, or between proof-schemas of the formal system, or between formulas and proof-schemas of the formal system, are all precisely defined notions, there can be corresponding relationships between the natural numbers that correspond to the formulas and proof-schemas of the formal system. Furthermore, if the correspondence is defined correctly, then if any such relationship between formulas/proof-schemas applies, then the corresponding relationship between natural numbers also applies. Also the inverse - if any relationship between natural numbers applies, then the corresponding relationship between formulas/proof-schemas also applies (note that if a natural number does not correspond to some symbol string of the formal system, then there can be no corresponding relationship, since there is not a corresponding formula/proof-schema).


Outline of a proof

The rest of the Part 1 of Gödel’s paper from the point “We now obtain an undecidable proposition…” is an outline description of a proof. But it is not an outline description of the method Gödel actually used in his proof, but of a different method. This has led to confusion for many people, because they are expecting the proof proper to follow that outline. For this reason, the reader is advised to ignore the rest of the introduction until one is fully cognizant with the actual proof that Gödel sets out in Part 2 of his paper.

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Diverse opinions and criticisms are welcome, but messages that are frivolous, irrelevant or devoid of logical basis will be blocked. Difficulties in understanding the site content are usually best addressed by contacting me by e-mail. Note: you will be asked to provide an e-mail address - any address will do, it does not require verification. Your e-mail will only be used to notify you of replies to your comments - it will never be used for any other purpose and will not be displayed. If you cannot see any comments below, see Why isn’t the comment box loading?.

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The Lighter Side


Paper on the diagonal proof

There is now a paper that deals with the matter of language and the diagonal proof, see On Considerations of Language in the Diagonal Proof.

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Other recently added pages

The Myths of Platonism


Goodman’s Paradox


The Platonist Rod paradox


The Balls in the Urn Paradox


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Lebesgue Measure

There is now a new page on a contradiction in Lebesgue measure theory.

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Easy Footnotes

I found that making, adding or deleting footnotes in the traditional manner proved to be a major pain. So I developed a different system for footnotes which makes inserting or changing footnotes a doddle. You can check it out at Easy Footnotes for Web Pages (Accessibility friendly).

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O’Connor’s “computer checked” proof

I have now added a new section to my paper on Russell O’Connor’s claim of a computer verified incompleteness proof. This shows that the flaw in the proof arises from a reliance on definitions that include unacceptable assumptions - assumptions that are not actually checked by the computer code. See also the new page Representability.

Previous Blog Posts

Moderate Platonism

Descartes’ Platonism

The duplicity of Mark Chu-Carroll

A John Searle Inanity

Man versus Machine

Fake News and Fake Mathematics

Ned Block’s Blockhead

Are we alone in the Universe?

Good Math, Bad Math?

Bishops Dancing with Pixies?

Artificial Intelligence

Cranks and Crackpots

The Chinese Room


For convenience, there are now two pages on this site with links to various material relating to Gödel and the Incompleteness Theorem


– a page with general links:

Gödel Links


– and a page relating specifically to the Gödel mind-machine debate:

Gödel, Minds, and Machines

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