Published online by Cambridge University Press: 20 November 2012
* My sincerest thanks to David Sedley, G.E.R. Lloyd, James Warren and Dunstan Lowe for their insightful comments and useful suggestions on earlier drafts of this paper. Any deficiencies are, of course, my own. Special thanks are also due to Eddy Kelsey and my parents, who indulged a young student's interest in ophthalmology.