### Ideas from 'Defending the Axioms' by Penelope Maddy [2011], by Theme Structure

#### [found in 'Defending the Axioms' by Maddy,Penelope [OUP 2013,978-0-19-967148-9]].

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###### 4. Formal Logic / F. Set Theory ST / 4. Axioms for Sets / j. Axiom of Choice IX
 17610 The Axiom of Choice paradoxically allows decomposing a sphere into two identical spheres
 Full Idea: One feature of the Axiom of Choice that troubled many mathematicians was the so-called Banach-Tarski paradox: using the Axiom, a sphere can be decomposed into finitely many parts and those parts reassembled into two spheres the same size as the original. From: Penelope Maddy (Defending the Axioms [2011], 1.3) A reaction: (The key is that the parts are non-measurable). To an outsider it is puzzling that the Axiom has been universally accepted, even though it produces such a result. Someone can explain that, I'm sure.
###### 5. Theory of Logic / C. Ontology of Logic / 3. If-Thenism
 17620 Critics of if-thenism say that not all starting points, even consistent ones, are worth studying
 Full Idea: If-thenism denies that mathematics is in the business of discovering truths about abstracta. ...[their opponents] obviously don't regard any starting point, even a consistent one, as equally worthy of investigation. From: Penelope Maddy (Defending the Axioms [2011], 3.3) A reaction: I have some sympathy with if-thenism, in that you can obviously study the implications of any 'if' you like, but deep down I agree with the critics.
###### 5. Theory of Logic / K. Features of Logics / 1. Axiomatisation
 17625 If two mathematical themes coincide, that suggest a single deep truth
 Full Idea: The fact that two apparently fruitful mathematical themes turn out to coincide makes it all the more likely that they're tracking a genuine strain of mathematical depth. From: Penelope Maddy (Defending the Axioms [2011], 5.3ii)
 17605 Hilbert's geometry and Dedekind's real numbers were role models for axiomatization
 Full Idea: At the end of the nineteenth century there was a renewed emphasis on rigor, the central tool of which was axiomatization, along the lines of Hilbert's axioms for geometry and Dedekind's axioms for real numbers. From: Penelope Maddy (Defending the Axioms [2011], 1.3)
###### 6. Mathematics / A. Nature of Mathematics / 4. The Infinite / g. Continuum Hypothesis
 17615 Every infinite set of reals is either countable or of the same size as the full set of reals
 Full Idea: One form of the Continuum Hypothesis is the claim that every infinite set of reals is either countable or of the same size as the full set of reals. From: Penelope Maddy (Defending the Axioms [2011], 2.4 n40)
###### 6. Mathematics / B. Foundations for Mathematics / 5. Mathematics as Set Theory / a. Mathematics is set theory
 17618 Set-theory tracks the contours of mathematical depth and fruitfulness
 Full Idea: Our set-theoretic methods track the underlying contours of mathematical depth. ...What sets are, most fundamentally, is markers for these contours ...they are maximally effective trackers of certain trains of mathematical fruitfulness. From: Penelope Maddy (Defending the Axioms [2011], 3.4) A reaction: This seems to make it more like a map of mathematics than the actual essence of mathematics.
###### 6. Mathematics / C. Sources of Mathematics / 4. Mathematical Empiricism / c. Against mathematical empiricism
 17614 The connection of arithmetic to perception has been idealised away in modern infinitary mathematics
 Full Idea: Ordinary perceptual cognition is most likely involved in our grasp of elementary arithmetic, but ...this connection to the physical world has long since been idealized away in the infinitary structures of contemporary pure mathematics. From: Penelope Maddy (Defending the Axioms [2011], 2.3) A reaction: Despite this, Maddy's quest is for a 'naturalistic' account of mathematics. She ends up defending 'objectivity' (and invoking Tyler Burge), rather than even modest realism. You can't 'idealise away' the counting of objects. I blame Cantor.