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 has been applied to the sequential order of the parts. The foundational papers on quantum mechanics have been arranged in a somewhat dif­ ferent manner. Chapters XVI-XIX are concerned with the logic of complementarity while in Chapters XX-XXII a more radical recon­ ceptualization is carried out. Two of the older papers (Chapters VI and VIII) have been revised to bring them more into line with present terminology. Other papers have been corrected by additions and omissions. Additions are marked by square brackets [ ], while double square brackets [[ II signify omis­ sions or parts to be omitted. Hence [[A]] [B] means that 'A' should be replaced by 'B'. The heading of one paper (Chapter XX) has been changed to make it more descriptive.