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### Proof-Theoretic Ordinal of ZFC or Consistent ZFC Extensions?

Let the proof theoretic ordinal $\alpha$ of a theory $T$ be the least recursive ordinal such that $T$ does not prove that $\alpha$ is well-founded. This ordinal is intended to quantify in some sense ...
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The set theory ETCS famously comes without the Replacement axiom schema (or an equivalent) that is part of ZFC. One (to me, not apparently useful) set that one cannot build in ETCS is $\coprod_{n\in \... 1answer 378 views ### Can the Burgess-Hazen analysis of Predicative Arithmetic be extended to Transfinite Types? Around page 300 of his book "Mathematical Thought and its Objects", Charles Parsons discusses the work of Edward Nelson, who believes that mathematical induction is impredicative, because it can be ... 10answers 4k views ### Is PA consistent? do we know it? 1) (By Goedel's) One can not prove, in PA, a formula that can be interpreted to express the consistency of PA. (Hopefully I said it right. Specialists correct me, please). 2) There are proofs (... 3answers 940 views ### Consistency of Analysis (second order arithmetic) Is there a proof of the consistency of Analysis (second order arithmetic), which is similar to Gentzen's proof of the consistency of arithmetic? Update: Which (different) methods can be used to ... 6answers 1k views ### Where in ordinary math do we need unbounded separation and replacement? [I have updated the question after initial comments in the hope of clarifying it.] I do quite a bit of reasoning, typically about topology and metric spaces, in "non-standard" foundations, such as ... 3answers 463 views ### Does the Feferman-Schutte analysis give a precise characterization of Predicative Second-Order Arithmetic? A definition is called impredicative if it involves quantification over a domain that contains the thing being defined. For instance, if you define hereditary property to be a property which applies ... 4answers 2k views ### Illustrating Edward Nelson's Worldview with Nonstandard Models of Arithmetic Mathematician Edward Nelson is known for his extreme views on the foundations of mathematics, variously described as "ultrafintism" or "strict finitism" (Nelson's preferred term), which came into the ... 1answer 1k views ### Martin's “Philosophical Issues about the Hierarchy of Sets” Some months ago (October 2010), in the context of the Workshop on Set Theory and the Philosophy of Mathematics, Professor Donald A. Martin gave a talk entitled "Philosophical issues about the ... 8answers 3k views ### ULTRAINFINITISM, or a step beyond the transfinite Cantor has, in the immortal words of D. Hilbert, given all of us a paradise (or perhaps, I would rather say, a great vacation spot), the TRANSFINITE.$\aleph_0, \aleph_1,\aleph_2\dots$the lists ... 4answers 1k views ### Subsystems of Peano arithmetic and incompleteness theorem I think everyone is familiar with Goedel's incompleteness theorems. In particular they imply that PA (Peano arithmetic) can not prove its own consistency. Now my question is what is the largest ... 1answer 388 views ### How many closed measure zero sets are needed to cover the real line? This question assumes familiarity with combinatorial cardinal characteristics of the continuum. Let$\mathcal{E}$be the$\sigma$-ideal generated by closed measure zero subsets of the real line. It ... 2answers 835 views ### Large cardinals without the ambient set theory? In an attempt to understand a bit better large cardinals, I have been thinking along the following lines, which could be summarized under the slogan Talk about cardinals without the (ambient) ... 0answers 545 views ### What is the role of the (formalized) omega rule in Ramified Analysis? In the 1960's, Feferman and Schutte did groundbreaking proof-theoretic work to find out the strength of predicative systems of second-order arithmetic. They used the ramified theory of types, a ... 1answer 271 views ### How many closed measure zero sets are needed to cover the real line, really? This is a refinement of an earlier question. This question assumes familiarity with combinatorial cardinal characteristics of the continuum. For the reader's convenience, I reproduce below the ... 2answers 445 views ### Did Gödel prove that the Ramified Theory of Types collapses at$\omega_1\$?

Second-Order Arithmetic is considered impredicative, because the comprehension scheme allows formulas with bound second-order variables that range over all sets of natural numbers, including the set ...