Gödel's First Theorem, that a certain well-formed formula, which is the code for:
Once the concept of proof has been made explicit,
and the criteria for being provable clearly laid down,
Gödel's theorem shows that there are truths which go
beyond that concept of provable.
``there are some mathematical disciplines that can never be completely formalized'' (J.Myhill, ``Some Implications of Mathematical Logic'',
The Review of Metaphysics,
VI, no.2, 1952, [pp.165-199, esp. pp.181-192,] p.182].
To be reasonable is not just to be in accordance with a rule.
Aristotle sometimes talks of
kata ton orthosn logon,
according to the right rule,
Gödel's theorem suggests that there is a real difference,
and that however carefully we specify rules or criteria,
reason is not just rule-observance and can go beyond mere conformity to rules.
Rules will still be important, but not all important.
Originality and creativity
In history or literary criticism it may be right to recognise the authority of great historians or sensitive critics without being able to reduce their reasoning to explicit syllogistic form.
In politics, although we may well distrust the judgement of men, we cannot have our government altogether a ``Government of laws, not men.''
It is difficult to see how there could be a model of Peano's postulates other than the standard one, which we pictured as the boy's railway track starting with some buffers and extending indefinitely far in one direction. This indeed is how the non-standard model starts too. The non-standardness only crops up over the horizon, so to speak, that is, among the transfinite numbers. The non-standard model of Peano Arithmetic starts off with a progression of order-type w, but then has indefinitely many additional chunks, each of order-type w* + w, the order-type of the integers, negative as well as positive. The order-type w* + w looks like this: .... -7, -6, -5, -4, -3, -2, -1, 1, 2, 3, 4, 5, 6, 7,.... it is what we use for dating, where 1 BC is followed immediately by 1 AD.
The chunks of order-type
w* + w
the non-standard model themselves have the order-type
eta, the dense order-type exemplified by the rational
There is a concertina effect.
Wherever we look, we find a stretch of track of
order-type w* + w, but between any two
such stretches, unless they are part of the same chunk,
and separated by only a finite number of pieces of rail,
there are further chunks, each of order-type w* + w.
That is to say, although locally we are dealing with a
discrete ordering, where each bit of track has a next
bit of track and a previous bit of track, globally the
separate chunks, each of order-type
w* + w, are not discrete; there is no next chunk after any particular one.
|+++. . . .++++. . . .++++. . . .++++. . . .++++. .. w w*+w w*+w w*+w w*+w
For fuller details, see G.S.Boolos and R.C.Jeffrey, Computability and Logic, Cambridge, 1974, 1980, 1989, Section 17, pp.193-195.
Peano's fifth postulate in first-order logic:
First-order logic is complete and compact. For every well-formed formula, G, of first-order logic,
Second-order logic is ontologically lush, not recursively axiomatizable, not analytic, but arguably adequate: first-order logic is economical with entities, can be done by computers, arguably analytic, but inadequate---even for characterizing the natural numbers.
Note: Self-reference only possible because there are a Dedekind infinity of natural numbers. Otherwise Gödel's coding impossible.
Click here for the text of a talk
I gave in Manchester in November 1996,
which was meant to be more accessible.