This paper has gone through a number of drafts, and an early version was first presented at the Australasian Philosophy Conference, Melbourne, July 1991. It has benefited from discussions with many people, and I am especially grateful to my colleagues at the ANU, RSSS, and to the staff and students at Rutgers University. I am much indebted to Georges Rey, for his invaluable comments on several drafts, and to Ruth Millikan for some very enjoyable and useful discussions during The Blizzard of ''93.