An earlier version of this material was delivered as the 2004 Ellen McArthur Lectures in the Faculty of History at the University of Cambridge. It was a great honour to be asked to give these lectures and I am grateful to the Trustees of the Fund for this invitation and for their hospitality during my stay in Cambridge. I have made substantial additions and alterations for the present text, but have attempted to maintain some of the informality of approach and greater freedom to express a personal opinion that was appropriate for an oral presentation.

My choice of subject may need some explanation. When I first pondered what theme I should take for the lectures, I realized that I had to choose between two dangers. I did not have any unpublished results waiting to be revealed. I could either select a topic on which I had already written, but at the risk that the response from my audience would be, ‘that was all very familiar, it's a pity he couldn't find anything new to say’. Or I could avoid this by lecturing on a subject on which I had done no previous research, at the risk of provoking the reaction, ‘that was all very derivative, it's a pity he didn't have anything of his own to contribute’.