This book explores this question, offering a novel insight into the nature of theory assessment itself. Dawid approaches the topic from a unique position, having extensive experience in both philosophy and high energy physics. He argues that string theory is just the most conspicuous example of a number of theories in high energy physics where non-empirical theory assessment has an important part to play. Aimed at physicists and philosophers of scienee, the book does not use mathematical formalism and explains most technical terms.