This book is a revised version of my D. Phil. thesis undertaken in Oxford and conferred as such in May 1994. Firstly, I would like to thank Prof. Wolfgang Wieland through whom my interest in the Topics was fITst aroused in one of his colloquiums at the University of Heidelberg and who was my first teacher in the study of Aristotle and Plato. The comments made by my two D.Phil. examiners, Mr Christopher Kirwan and Prof. Mario Mignucci were of great help. In addition, the suggestions of the publisher's anonymous reader helped make the thesis more readable. I am grateful to the British Academy·for enabling me to study for a D. Phil. at Oxford; to the Studienstiftung des deutschen Volkes which supported me throughout studentship; and to the editors of Philosophia antiqua for accepting my thesis for publication.