### Ideas from 'On the Question of Absolute Undecidability' by Peter Koellner [2006], by Theme Structure

#### [found in 'Philosophia Mathematica' (ed/tr -) [- ,]].

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###### 4. Formal Logic / F. Set Theory ST / 1. Set Theory
 17884 Mathematical set theory has many plausible stopping points, such as finitism, and predicativism
 Full Idea: There are many coherent stopping points in the hierarchy of increasingly strong mathematical systems, starting with strict finitism, and moving up through predicativism to the higher reaches of set theory. From: Peter Koellner (On the Question of Absolute Undecidability [2006], Intro)
 17893 'Reflection principles' say the whole truth about sets can't be captured
 Full Idea: Roughly speaking, 'reflection principles' assert that anything true in V [the set hierarchy] falls short of characterising V in that it is true within some earlier level. From: Peter Koellner (On the Question of Absolute Undecidability [2006], 2.1)
###### 5. Theory of Logic / K. Features of Logics / 5. Incompleteness
 17894 We have no argument to show a statement is absolutely undecidable
 Full Idea: There is at present no solid argument to the effect that a given statement is absolutely undecidable. From: Peter Koellner (On the Question of Absolute Undecidability [2006], 5.3)
###### 6. Mathematics / A. Nature of Mathematics / 5. The Infinite / i. Cardinal infinity
 17890 There are at least eleven types of large cardinal, of increasing logical strength
 Full Idea: Some of the standard large cardinals (in order of increasing (logical) strength) are: inaccessible, Mahlo, weakly compact, indescribable, Erdös, measurable, strong, Wodin, supercompact, huge etc. (...and ineffable). From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.4) A reaction: [I don't understand how cardinals can have 'logical strength', but I pass it on anyway]
###### 6. Mathematics / B. Foundations for Mathematics / 4. Axioms for Number / d. Peano arithmetic
 17887 PA is consistent as far as we can accept, and we expand axioms to overcome limitations
 Full Idea: To the extent that we are justified in accepting Peano Arithmetic we are justified in accepting its consistency, and so we know how to expand the axiom system so as to overcome the limitation [of Gödel's Second Theorem]. From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.1) A reaction: Each expansion brings a limitation, but then you can expand again.
###### 6. Mathematics / B. Foundations for Mathematics / 4. Axioms for Number / g. Incompleteness of Arithmetic
 17891 Arithmetical undecidability is always settled at the next stage up
 Full Idea: The arithmetical instances of undecidability that arise at one stage of the hierarchy are settled at the next. From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.4)