This provocative new book attempts to resolve traditional problems of identity over time. It seeks to answer such questions as "How is it that an object can survive change?" and "How much change can an object undergo without being destroyed?" To answer these questions Professor Heller presents a completely new theory about the nature of physical objects and about the relationship between our language and the physical world. According to his theory, the only actually existing physical entities are what the author calls "hunks," four dimensional objects extending across time and space. This is a major new contribution to ontological debate and will be essential reading for all philosophers concerned with metaphysics.