### What we talk about when we talk about numbers

*Annals of Pure and Applied Logic*

In this paper, I describe and motivate a new species of mathematical structuralism, which I call *Instrumental Nominalism about Set-Theoretic Structuralism*. As the name suggests, this approach takes standard Set-Theoretic Structuralism of the sort championed by Bourbaki and removes its ontological commitments by taking an instrumental nominalist approach to that ontology of the sort described by Joseph Melia and Gideon Rosen. I argue that this avoids all of the problems that plague other versions of structuralism.

### Review of John P. Burgess’ *Rigor and Structure*

*Philosophia Mathematica *24(1):129-136

In this review, I focus on the possibility of giving a precise account of informal mathematical proof; and the lesson that Burgess draws from the indifference that mathematicians have towards questions about the subject matter of their discipline.

### Indispensability arguments and instrumental nominalism

*Review of Symbolic Logic* 5(4):687-709

In the philosophy of mathematics, indispensability arguments aim to show that we

are justified in believing that mathematical objects exist on the grounds that we make indispensable reference to such objects in our best scientific theories (Quine, 1981a; Putnam, 1979a) and in our everyday reasoning (Ketland, 2005). I wish to defend a particular objection to such arguments called instrumental nominalism. Existing formulations of this objection are either insufficiently precise or themselves make reference to mathematical objects or possible worlds. I show how to formulate the position precisely without making any such reference. To do so, it is necessary to supplement the standard modal operators with two new operators that allow us to shift the locus of evaluation for a subformula. I motivate this move and give a semantics for the new operators.

### Two types of abstraction for structuralism (with Øystein Linnebo)

*Philosophical Quarterly *64(255):267-283

If numbers were identified with any of their standard set-theoretic realizations, then they would have various non-arithmetical properties that mathematicians are reluctant to ascribe to them. Dedekind and later structuralists conclude that we should refrain from ascribing to numbers such ‘foreign’ properties. We first rehearse why it is hard to provide an acceptable formulation of this conclusion. Then we investigate some forms of abstraction meant to purge mathematical objects of all ‘foreign’ properties. One form is inspired by Frege; the other by Dedekind. We argue that both face problems.

### Category theory as an autonomous foundation (with Øystein Linnebo)

*Philosophia Mathematica* 19(3):227-254

Does category theory provide a foundation for mathematics that is autonomous with respect to the orthodox foundation in a set theory such as ZFC? We distinguish three types of autonomy: logical, conceptual, and justificatory. We argue that, while a strong case can be made for its logical and conceptual autonomy, its justificatory autonomy turns on whether or not mathematical theories can be justified by appeal to mathematical practice. If they can, a category-theoretical approach will be fully autonomous; if not, the most natural route to justificatory autonomy is blocked.

### Aristotle on the subject matter of geometry

*Phronesis* 54: 239-260

I offer a new interpretation of Aristotle’s philosophy of geometry, which he presents in greatest detail in *Metaphysics* M 3. On my interpretation, Aristotle holds that the points, lines, planes, and solids of geometry belong to the sensible realm, but not in a straightforward way. Rather, by considering Aristotle’s second attempt to solve Zeno’s Runner Paradox in Book VIII of the *Physics*, I explain how such objects exist in the sensibles in a special way. I conclude by considering the passages that lead Jonathan Lear to his fictionalist reading of *Met.* M3,1 and I argue that Aristotle is here describing useful heuristics for the teaching of geometry; he is not pronouncing on the meaning of mathematical talk.

### Platonism and Aristotelianism in Mathematics

*Philosophia Mathematica *16(3): 310-332

Philosophers of mathematics agree that the only interpretation of arithmetic that takes that discourse at ‘face value’ is one on which the expressions ‘N’, ‘0’, ‘1’, ‘+’, and ‘×’ are treated as proper names. I argue that the interpretation on which these expressions are treated as akin to free variables has an equal claim to be the default interpretation of arithmetic. I show that no purely syntactic test can distinguish proper names from free variables, and I observe that any semantic test that can must beg the question. I draw the same conclusion concerning areas of mathematics beyond arithmetic.