In this book I have attempted to give an account of some of the most im- portant of Whitehead's philosophical writings - his writings on the philoso- phy of science as well as his metaphysics. I have tried to show that although there are novelties in Whitehead's later philosophy there are also continuities with his earlier work in the philosophy of science. For a more detailed account of Whitehead's metaphysics, I would refer the reader to my book The Philosophy of Whitehead (The Muirhead Library of Philosophy), Allen and Unwin, London 1959 (Collier Books, New York 1962). On the whole I believe my view of Whitehead in that work, at least as far as his metaphysics is concerned, is not materially different from that held in the present one, although there are some differences in emphasis and interpretation. I wish to thank the administrateur delegue of the Revue Internationale de Philosophie, Brussels; the publishers Allen and Unwin, London; and Sprin- ger-Verlag, Heidelberg, for kindly giving me permission to publish amended versions of the following papers which I originally published with them: "Whitehead and the Idea of Equivalence" Revue Internationale de Philoso- phie, No. 56-57, 1961, pp. 167-184; "The Relevance of "On Mathematical Concepts of the Material World" to Whitehead's Philosophy", in The Relevance of Whitehead. Ed. Ivor Leclerc (The Muirhead Library of Philo- sophy) Allen and Unwin 1961, pp. 235-260; "Whitehead and the Philosophy of Time". Studium Generale. Springer-Verlag 1970, pp.