It is a pleasure to thank Alan Kay and all the other members of the Learning Research Group at Xerox Palo Alto Research Center, both for help with this research and for providing a congenial and stimulating place in which to work. I would also especially like to thank my adviser, Terry Winograd, for his many helpful insights and patient advice. Among the other people who have aided in this effort, I would like to express my gratitude to Giuseppe Attardi, Danny Bobrow, Hjohn Seely Brown, Peter Deutsch, Carl Hewitt, Ken Kahn, Henry Lieberman, David Shaw, Gerry Sussman, Bert Sutherland, and Gio Widerhold. The Xerox Corporation generously provided support for doing this research.