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first-order and higher-order logic, model theory, set theory, proof theory, computability theory, formal languages, definability, interplay of syntax and semantics, constructive logic, intuitionism, philosophical logic, modal logic, completeness, Gödel incompleteness, decidability, undecidability, theories of truth, truth revision, consistency.

17 votes
2 answers
1k views

Is it consistent with ZFC that the real line is approachable by sets with no accumulation po...

Let $P$ denote the following proposition: There exists a set $S$ of subsets of $\mathbb{R}$ such that $S$ is totally ordered by inclusion; each member of $S$ has no accumulation points; the union of …
Julian Newman's user avatar
11 votes
3 answers
933 views

Some "axiom of choice" and "dependent choice" issues

I am probably about to ask some fairly basic questions, and yet I have found it quite hard to find the answers to these. If I understand correctly, mathematicians tend to be quite happy working with …
Julian Newman's user avatar
8 votes
3 answers
690 views

Is "the purely probabilistic version of Freiling's axiom of symmetry" disprovable in ZFC?

I'm trying to pinpoint the "intuitive argument" for Freiling's Axiom of Symmetry. It's meant to be a "probabilistic" argument, so thinking about what seems to me to be the probabilistic intuition, it …
Julian Newman's user avatar
6 votes
1 answer
714 views

Which part(s) of this proof of Goodstein's Theorem are not expressible in Peano arithmetic?

EDIT: Noah Schweber helpfully points out that $\mathsf{ACA}_0$ is a conservative extension of Peano arithmetic in which certain aspects of my proof not expressible in Peano arithmetic are expressible. …
Julian Newman's user avatar
1 vote
2 answers
327 views

Heuristic interpretations of the PA-unprovability of Goodstein's Theorem

I've relatively recently learned about Goodstein's Theorem and its unprovability in Peano arithmetic (the Kirby-Paris Theorem). I do not have any real knowledge of formal logic; but I think I've seen …
Julian Newman's user avatar