This book is in the tradition that defines the philosophical center of contemporary philosophy of science, the tradition of Carnap, Hempel, and Nagel as supplemented by generous additions from Austin and an Oxfordized Wittgenstein in the style introduced by N. R. Hanson. This tradition has been criticized both from the philosophical left, by Sellars, and from the philosophical right, by Bergmann. Achinstein's work is so squarely in the center that neither Sellars nor Bergmann ever appear in the index. That makes the book the third or fourth generation of a series in that central tradition all of which have been concerned largely with discussing their predecessors in that same tradition. This series is now producing books—Achinstein's is one of them—that have the virtues and the defects of an ingrown aristocracy: finely shaped features; an aloofness with respect to which no doubt is felt that it is justified ; and in their quarrels among themselves the cunning of foxes; but with none of the robust life and vitality of lions that first secured them the right to rule.