This book argues against the mainstream view that we should treat propositional attitudes as internal states, suggesting that to treat beliefs as things of certain sort (i.e. to reify them) is a mistake. The reificatory view faces several problems that the non-reificatory view avoids, and it is argued the non-reificatory view is more faithful to the everyday concept of belief. There are several major reasons why it might be thought that a reificatory approach to mental states is nevertheless unavoidable, but this book attempts to show that none of these reasons is at all convincing; in each case, the evidence is consistent with a non-reificatory view. Having argued that the popularity of the reificatory view is unjustified, the author examines history of psychology and philosophy of mind, and the structure of psychological language, in order to show that this popularity is quite understandable, but mistaken nonetheless.