• Pippen
    80
    I always wondered how to describe Gödel's Incompleteness Theorems in a nutshell, without all the technicalities, but still very close to what its idea is. I am interested to read if my description nails the idea of the Incompleteness Theorems or where I commit serious errors or confusions.

    p.s. I did this a while ago already, but I think my new version is better than the old one. Please only answer if you somehow familiar with the theorems!

    First Incompleteness Theorem

    We assume a consistent formal system S where we can syntactically correct formulate the following statement G: G <-> ~Proof(G). There are two cases within S:

    (1) G is provable, but then G is not provable (~Proof(G)) which is a contradiction and therefore impossible,

    (2) ~G is provable which means (~G & Proof(G)) v (G & ~Proof(G)), but that means in either case G will be proven which is a contradiction and therefore impossible.

    So our (consistent) system S cannot prove G or ~G and is therefore incomplete (or it could prove G or ~G if it was inconsistent for trivial reasons). Within S we can't decide if G or ~G is true, but of course from a meta-view we know that G is true. Gödel's "only" accomplishment was to show that G can be formulated syntactically correct in a special S called PM and therefore "infects" whole math (and yes, that was genius).

    Second Incompleteness Theorem

    Let's assume our system S again, this time strong enough to prove its First Incompleteness Theorem, i.e. if S is consistent then G which says G <-> ~Proof(G). Let's assume we could prove the consistency of S in S. Then by mp we could prove in S that G which is impossible due to the First Incompleteness Theorem, therefore the assumption must be false.
  • Deleted User
    0
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  • Pippen
    80
    Hi Tim,

    I add "If S is consistent then G (is not provable)" to S as an axiom. It then becomes clear why we can't prove the consistency of S within S because it would lead to a proof of G which is impossible by the First Incompleteness Theorem.
  • Deleted User
    0
    This user has been deleted and all their posts removed.
  • A Seagull
    615
    My take on Gödel's incompleteness theorems is that they start with an initial assumption of completeness - ie that every statement that relates to a specific system S is either true or false. The proof (which incorporates basic maths) then leads to an inconsistency. Conclusion: no system that includes basic mathematics is complete.
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