This book consists of five of the Woodbridge Lectures delivered at Columbia University in 1983. A sixth lecture," Causation and Explanation," somewhat remote in theme and treatment from the others is not included. I wish to thank the members of the philosophy department at Columbia for their invitation to deliver these lectures and for the pleasure and stimulus of their company and comments during my stay in New York. The lectures were originally composed in Oxford during the early months of 1980 and were subsequently delivered in that University. Though a good many of the thoughts they contain have been given, sometimes fuller, sometimes abbreviated, expression in others of my articles and reviews, they have not before been brought together and presented in print in their present form. The original composition of the lectures was prompted by a growing sense of a certain unity in the approaches, which I found plausible or appealing, to several apparently disparate topics; and by a hope, no doubt delusive, that some persistent philosophical tensions might be eased by an exposure of the parallels and connections between these approaches.