Mancosu - Philosophy of Mathematical Practice (Oxford, 2008).pdf

272 jamie tappendenopponents aggressively rejected the axiom while finding themselves repeatedlyapplying it tacitly in proofs.) Peacocke has identified a deep problem that wehave only begun to see clearly. Here too, philosophical analysis and study ofrich cases from practice will have to go hand in hand.(c) Wright/Hale on the Caesar problemThe insight inspiring neo-logicism is that arithmetic can be derived from anabstraction principle. This suggests that we revisit Frege’s reservation: Howcan we conclude, from this definition, that Julius Caesar is not a number?One appealing strategy is to spell out and defend the naive reaction: Caesarisn’t a number because he just isn’t the kind of thing that can be. Pursuingthis line, Hale and Wright (2001) argue that in sortal classification somecategories are inherently disjoint. They concentrate their brief discussion onabstract reflections on individuation and criteria of identity; my impression isthat most readers have been left wanting more. A good way to go beyondfoot-stomping staredowns where each side just reaffirms favoured intuitionsis to assemble more information about a range of cases. The neo-logicistprogramme may need clarification of the structure of natural classification inmathematics for a further reason: the so-called ‘bad company objection’. Someabstraction principles have evidently false consequences, but might be excludedif a viable artificial/natural distinction (‘Cambridge’/‘non-Cambridge’ inWright/Hale’s parlance) could be articulated and defended. Not all thetroublesome abstracts are plausibly ruled unnatural, so this is not a cureall.But we shouldn’t assume that all problematic abstracts are problematic forthe same reason.9.6 Summing up: the Port Royal principleThe examples we’ve seen are enough to conclude that mathematical defining isa more intricate activity, with deeper connections to explanation, fruitfulness ofresearch, etc. than is sometimes realized. We need a philosophical frameworkto help us keep the books while we sort it out, and to guide us as towhat consequences if any this can have for other areas of philosophy. Thephilosophical stance implicit in the above has affinities to the position thatPenelope Maddy ((1997) and elsewhere) has called ‘mathematical naturalism’.Though I differ with Maddy on some details, we agree that rich analysisof mathematical practice is a sine qua non for judgements in the philosophyof mathematics. The lines that open this paper, from the Port Royal Logic’streatment of real definition, prompt the name ‘Port Royal principle’ for the kind