When we talk about your application, there are essentially two contexts, or worlds. The first is the inside world. This is anything that lives inside of your application state. To be even more strict, it's anything in your application state that can also be serialized. This doesn't mean that your state has to be capable of being serialized to JSON or similar, but that pieces that are serializable are the inside world parts of your application. Things in the inside world are easily used, manipulated, and tested through actions.