The central models of theoretical physics have been extraordinarily successful in describing and predicting the behaviour of physical systems under an enormous range of conditions. But why are these mathematical theories so successful, and how is their structure influenced by the nature of the observations on which they are inevitably based? This intriguing book examines these subtle and fundamental issues, and concludes with the assertion that successful prediction provides compelling support for belief in a world independent of the observer. Dealing with important and basic aspects of the general framework of physics, this book will appeal to undergraduate and graduate students in the physical sciences, and to anyone with an interest in the empirical and metaphysical foundations of science.