1 It is wonderful for me to have the opportunity of paying tribute to Kenneth Arrow, who is not only one of the greatest economists of our time, but also one of the finest thinkers of our era. That itself makes the occasion very special for me, but on top of that, it is marvellous that I have the company of Eric Maskin, with whom I used to teach a much enjoyable joint course on social choice theory at Harvard, until he deserted us for the Institute for Advanced Study. And it is very pleasing for me to have Joe Stiglitz as the participating chair of the meeting (having known Joe for many years, I could assure the audience that there was no danger of Joe being an aloof chair), and it is very good to know that Akeel Bilgrami's intellectual vision is behind the planning of this event. I am in admirable company, and I begin by expressing my appreciation of that, but most especially by thanking Ken Arrow himself, for making us all think in new lines, and personally for me, for being such a major influence on my own intellectual life. I shall be particularly concerned in this lecture with Arrow's path-breaking "impossibility theorem," for which Arrow managed to find, in line with his sunny temperament, a rather cheerful name: "General Possibility Theorem." This result, and with it the formulation of the demands of mathematical social choice theory, were real watersheds in the history of welfare economics as well as of voting theory and collective choice. The informational foundation of modern social choice theory relates to the basic democratic conviction that social judgments and public decisions must depend, in some