The chapters in this book are based on lectures given by their authors as part of the University of Cambridge International Summer Schools in July 1996 and July 1997. Some 300 people of all ages and from over thirty countries take part in these interdisciplinary programmes each year, whilst a further 850 attend a range of concurrent subject-specialist Summer Schools. Those attending the International Summer School spend a month in Cambridge, living in the colleges and gathering in smaller groups for classes three times a day. In addition to classroom sessions and private study, the entire group is invited to attend a plenary lecture each day.

In my capacity as Director of the Summer School, I arrange a series of plenary lectures which will stimulate this audience for an hour each morning and encourage its members to think about things beyond their immediate course of study. I am struck each year by the quality of these lectures, and regret that more people cannot benefit from their preparation. For the past few years these plenary lectures have been open to members of the university, and selected lectures from the 1993 series were published as Cambridge Minds. This volume follows both that example and, to a certain extent, a much earlier one: the lectures on astronomy which James Stuart delivered to the workmen of Crewe in the summer of 1868 were written up by him and published by the University Press at the end of 1869, so that a permanent record might be kept and enjoyed.