This book is a selection from the articles that I have written over a period of more than twenty years. Since the focus of my research interests has shifted several times during this period, it would be difficult to identify a common theme for all the papers in the volume. Following the Swedish tradition, I therefore present this as a smoergasbord of philosophical and cognitive issues that I have worked on. To create some order, I have organized the sixteen papers into five general sections: (1) Decision theory; (2) belief revision and nonmonotonic logic; (3) induction; (4) semantics and pragmatics; and (5) cognition and evolution. Having said this, I still think that there is a common theme to my work over the years: The dynamics of thought. My academic interests have all the time dealt with aspects of how different kinds of knowledge should be represented, and, in particular, how changes in knowledge will affect thinking. Hence the title of the book.