The original, Dutch version of this book served in 1979 as a doctoral disserta- tion in philosophy at the Free University in Amsterdam. In this preface to the - slightly revised - English translation, I wish once again to express my gratitude to my supervisors, Prof. J. van der Hoeven of the Free University and Prof. G. Nuchelmans of the University of Leiden, for their excellent and stimulating support. Professor van der Hoeven was associated with this project from the outset. It was a privilege to benefit from his incisive commentaries, especially in those instances where the objective was...