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 to break through to more fundamental insights. I shall not lightly forget his friendly and heartening encouragement. I am equally grateful for my discussions with Professor Nuchelmans. I almost always tried to follow his advice, since it was based upon awesome expertness and erudition. I am happy to have found in the person of Herbert Donald Morton, Th.M., M.A., an able and enthusiastic translator. Drs. Gerben Groenewoud made the translations of a number of the Latin citations. I acknowledge permission from Routledge and Kegan Paul and the University of Toronto Press to quote from The Collected Works of John Stuart Mill. And I thank the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) for fmancing this translation.