These lectures have been edited to suit them to the printed, rather than the spoken word. With the exception of a few minor additions the text is substantially the same as that of the transcripts taken from the original tape recordings.The questions which stimulated the material content have had to be subjected to the same selection and compression without, I hope, too much misrepresentation of the intentions of the participants. To them I would like to express my thanks; also to Frank Philips, whose arduous and spontaneous translation made the lectures in São Paulo possible. Of my wife's contribution in changing the spoken to the printed word I cannot speak highly enough. This book has cost her many hours of skilful work.