'At the frontiers of physics and cosmology theoretical speculation proceeds without the constant confrontation with experiment often thought to be required for scientific validation. Richard Dawid explores this issue in the case of string theory. He offers a fascinating new perspective on non-empirical theory assessment, based on the concept of scientific underdetermination. I heartedly recommend this book to both physicists and philosophers.' David Gross, Kavli Institute for Theoretical Physics, and Nobel Laureate in Physics, 2004