In this book, Eli Hirsch focuses on identity through time, first with respect to ordinary bodies, then underlying matter, and eventually persons. These are linked at various points with other aspects of identity, such as the spatial unity of things, the unity of kinds, and the unity of groups. He investigates how our identity concept ordinarily operates in these respects. He also asks why this concept is so cental to our thinking and whether we can justify seeing the world in terms of such a concept.This is the revised and updated edition of a hardback published in 1982.