Would anyone happen to have a recommendation for someone hoping to make Gödel's incompleteness theorem "click"? It feels like every time I reapproach it, I have to start the intuition building all over again.
Why are you interested? I recommend the short book "Godel's proof".
Godel's work is wrapped up in historical context which is both interesting and distracting from the core idea. For example, Bertrand's Russells' work and book isn't really essential, it's just the system which Godel worked in to do his proof.
The corresponding section in the beginning of Scott Aaronson’s “Quantum computing since Democritus” (the online version works[1]; see also the related blog post[2]). It’s short and to the point (unlike GEB) and enough to understand why something in this vein must be true. It’s not detailed enough, though, to understand each of the technical conditions in the standard statement individually (no explanation of ω-consistency and the like).
Is there an entry-level explanation that explicitly goes over the following points? Let me illustrate:
>The Incompleteness Theorem says that, given any consistent, computable set of axioms, there's a true statement about the integers that can never be proved from those axioms.
Upon reading something like this I immediately have questions like: if this is so, then how do we know that this statement about the integers is true at all? What does it mean for something to be true within a set of axioms when you can't prove it? Why don't we say that the truth of this statement, within those axioms, is undetermined? If we, on the outside, know that it's true, why can't we forcefully plug that truth back into the theory?
OK, I know that last one, you can do it but then you can also do an incompleteness proof for the new theory. But still, if the "problem" is only with self-referential statements, why can't we somehow isolate all self-referential statements and have a theory that's complete and consistent except for some caveats, which seems vastly better than just inconsistent, period?
Sorry if that makes no sense, I know this topic is famous for attracting cranky discourse.. It just feels like all the popular explanations stop just short of really grappling with the real weirdness of the theorem.
Right, so definitely check Aaronson, the entirety of what I linked is a couple dozen pages at most.
But the statement you quoted, while true, is deliberately provocative. There are two standard statements:
- The one for the First Incompleteness Theorem implies the existence of a statement P independent from the theory, i.e. one that can be neither proved nor disproved using it. (That is, theory proves false iff theory + P proves false iff theory + not-P proves false.) Here we don’t opine on whether P is true or not.
- The one for the Second Incompleteness Theorem points to a specific statement: the consistency of the theory (that you can state that within the theory itself is nontrivial, but at the same time much of the point of and the reason for the whole hubbub). Here we know it’s true because we assumed it! That is, in the metalogic which we’re (implicitly, unless you dive in very deep) using to reason about all this, we have the disjunction:
- Either the theory proves false, then it proves anything at all (boring);
- Or it does not prove false, then it does not prove th(e true statement th)at it does not prove false.
Why we can’t just cut out self-referential stuff in our logic... for the same reason we can’t (usefully) just cut out infinite loops in our programming language, I think: it doesn’t take much to have either of those. The statements are not self-referential in any exotic way. What you need is—AFAIU—for the theory to be able to be able to talk about (at least in terms of halting or not) programs in a Turing-complete language, a proof verifier for the theory written in that language and an interpreter for itself in it. Encoding all of that in Peano arithmetic is predictably awful (especially if your name is Gödel, you have never seen a programming language, and Turing completeness hasn’t been invented yet). But none of it is exactly advanced functionality for a logic system; the self-interpreter isn’t even a part of it, it’s just something all Turing-complete languages can do. (This part is more wishy-washy than I’d like, but that’s what you get for asking an amateur.)
> all the popular explanations stop just short of really grappling with the real weirdness of the theorem.
No offense, but if I’m reading your comment correctly you’re making it out that nobody familiar with the proof has ever considered what “truth” really is. That’s…well, there’s a saying amongst physicists that, “you’re not even wrong.” The semantics of language and math have a copious amount of literature behind them. Not to mention that even asking the question is, forgive me, a tad juvenile.
Also, recursively applying known unknowns back into the statement (? If I understood that correctly) is itself incomplete: how could a system be “complete” if there are unknowns?
Forgive me if it seems I, too, have ventured into the cranky side of the discourse.
Speaking as a PhD in pure math: It's a great book, but I would not recommend it at all for someone trying to get the gist of the theorem. GEB is a book for those who already have a strong self-interest in or prediliction to the intricacies of mathematical logic. It's a long book and is likely to be off-putting for those wanting a practical approach.
Raymond Smullyan's book (another reply to the OP) is a much better choice.
Fair point. I don't have a mathematical grounding, but found it a thought-provoking and accessible intro. But different people will find different approaches compelling.
"Every few decades an unknown author brings out a book of such depth, clarity, range, wit, beauty and originality that it is recognised at once as a major literary event. This is such a work."
Nevertheless, Infinity and the Mind: The Science and Philosophy of the Infinite by Rudy Rucker is, in my opinion, a better choice for the mathematically inclined layman interested in Goedel's discoveries and plenty of related mathematics. Rucker's book even includes an account of his (somewhat over-enfusive) meeting with the great man.
