Patricia T. O’Conner and Stewart Kellerman, filling in for the vacationing William Safire, make the case for my beloved singular they in the On Language column in The New York Times Magazine:
The idea that he, him and his should go both ways caught on and was widely adopted. But how, you might ask, did people refer to an anybody before then? This will surprise a few purists, but for centuries the universal pronoun was they. Writers as far back as Chaucer used it for singular and plural, masculine and feminine.