This book concerns belief. Although this book is a scientific book, and not a popular book, it aims to present the scientific topics in a simple and fascinating manner, so that everybody can read it and understand the presented concepts. The first part of the book introduces the reader to the basic concepts of the belief science. The second part considers belief engineering, precisely: how to use the concepts of belief science in order to help solve diagnostic problems? Although the topics are examined with scientific rigor, the style of their presentation is made more challenging by means of the interactive presence of a virtual character that will accompany the reader during a scientific adventure: a nine days journey to discover the belief science. The book is useful to students and researches who want to approach the basic concepts of Bayesian networks and probabilistic expert systems in an easy way. It is also a pleasant reading for everyone who wants to discover the beauty of the scientific method applied to the mental state "belief".