In selecting the papers for this volume I have excluded all physics papers proper. I have further omitted all book rev.iews. Instead, I have included two papers not published previously; they are marked by an asterisk (*) in the table of contents. Since many of the papers were occasioned by Symposia or similar gatherings their chronological order is rather accidental. Hence I have tried to group the papers thematically into four parts. Within each part the order of sequence is from the more general to the more special, or from a more popular to a more technical treatment. The same principle...