# 1. Something Exists The minimal starting point of physical description is the bare fact that something exists and is seen to change. We notice change because we register differences: a scratch on a table, a shifted shadow, a moved object. Call this something $u$. Its presence can be at least partially described relative to itself by writing $u(\mathbf{r})$, where $\mathbf{r}$ is a label for relative position within the extent of $u$: $$ u(\mathbf{r}) \geq 0. $$ The name we give to $u$ is *energy*. If apparently distinct things exist and interact, they are not fundamentally independent substances. They are configurations of $u$. Interaction is therefore always only between $u$ and itself. In this book, $u$ names that one interacting substance: all that physically exists.[^existence] This single assumption - that something real is present as a nonnegative magnitude across its extent - is the ontological foundation of all known physics. Nothing more primitive is assumed in what follows. [^existence]: There cannot, by definition, be another kind of existence if it is to interact with the physical.