His best-known work is probably Frege's Conception of Numbers as Objects (1983), where he argues that Frege's logicist project could be revived by removing Basic Law (V) from the formal system. Arithmetic is then derivable in second-order logic from Hume's principle. He gives informal arguments that (i) Hume's principle plus second-order logic is consistent, and (ii) from it one can produce the Dedekind-Peano axioms. Both results were later to be proven more rigorously by George Boolos and Richard Heck.
He recently co-edited the Blackwell Companion to the Philosophy of Language, with Bob Hale.