This is not an easy paper to write or to deli ver. The only possible reason for asking me to deli ver it is that I happen to be in America this year and you may want to see a new face. Among the participants at this conference are many who are eminently qualified to give a paper on this very subject. Worse still, some of them have in fact done so, and many of the participants, myself inc1uded, have had the benefit of reading their work.