We are grateful to Dr. Chen Yehezkeli and Professor Ian Jarvie for their comments on the penultimate draft;

to Aaron Agassi for his many suggestions for improvements to the presentation; to Maggie Meitzler and her staff for the highly professional copyediting; and to Beatrice Rehl, philosophy editor for Cambridge University Press, and her read­ers for their encouragement and their penetrating and very helpful critical comments.