Rationality and Compulsion presents a unique examination of mental illness - derived from philosophical action theory. Delusion is common to many mental disorders, resulting in actions that, though perhaps rational to the individual, might seem entirely inappropriate or harmful to others. So what is it that causes these actions, and why do they continue? The theory expounded in this book shows how the key to this problem might be compulsion. This book presents a new analysis of the notion of compulsion - developed from action theory. The books starts with an introduction to action theory (for the benefit of non-philosophers). It then shows how insights from action theory can help us better understand mental illness, before developing an analysis of compulsion that emphasizes the element of unavoidability. The book argues that what is fundamentally disturbing to the person suffering from delusion is not so much the fact that the disorder tends to lead to irrational actions but rather the fact that he or she is unable to avoid performing these actions. The individual is or feels compelled to act in the way he or she does. The book contains some concrete illustrations of this idea as applied to several psychiatric diagnoses, such as paranoia, phobia, and psychopathy.