# Preface There is an old observation, so obvious it is easy to overlook: for two things to interact, they must share something that can be mutually affected. The fact that we register interaction at all implies a shared substrate. Throughout this book, we call that substrate **energy**. All that is required to recover known physics is the concept of energy. Static energy, in the sense developed in what follows, could not be experienced. What reaches us are the effects of the redistribution of energy. By registering those effects we infer its continuous flow. This is not a limitation to be overcome. What we have access to is the consequence of its flow. And it turns out that is enough. The pages that follow ask a single question, pursued with as few assumptions as possible: if something exists and redistributes continuously, what must follow? The answer is, as we shall see, at least all of known physics.