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.