For a meaningful understanding of physics, it is necessary to realise that this corpus of knowledge operates in a register different from natural thought. This book aims at situating the main trends of common reasoning in physics with respect to some essential aspects of accepted theory. It analyses a great many research results based on studies of pupils and students at various academic levels, involving a range of physical situations. It shows the impressive generality of the trends of common thought, as well as their resistance to teaching. The book's main focus is to underline to what extent natural thought is organised. As a result of this mapping out of trends of reasoning, some suggestions for teaching are presented; these have already influenced recent curricula in France. This book is intended for teachers and teacher trainers principally, but students can also benefit from it to improve their understanding of physics and of their own ways of reasoning.