160 Personal Content Experience: Managing Digital Life in the Mobile Age strictly defi nes a set of metadata schemas, while also allowing creation of new fl exible metadata defi nitions for any purpose. 5.6.1 Instance Metadata and the Schema Each metadata attribute has semantics associated with it in the form of ontology. Our ontology is not static or something used in designing the framework. It lives in the framework and our framework therefore contains two kinds of metadata. There is the actual metadata describ- ing the content objects, referred to as instance metadata, and metadata 21 about metadata, that is, our ontology or schema. All instance metadata – tags, relationships, context, and usage history – is stored in the database as instances of ontology. The frame- work also stores and manipulates the schema, which allows adding or changing new metadata schemas at runtime. In theory, the semantics of next to everything can be changed. For instance, a developer can declare that from this point onwards, the attribute singer is no longer a valid metadata attribute for songs, but is used to describe Norwegian oil paintings from the 19th century. Obviously, that would result in havoc among all old applications. Therefore, in facing reality, we have limited the ways semantics can be changed. Usually adding new metadata and new meanings is simple and permitted, but modifying existing metadata defi nitions is not allowed. 5.6.2 Initializing the Framework When our framework in turned on for the fi rst time, it is completely empty. There is nothing, no ontology, that is to say, the ontology is saying that nothing can exist; it is void. You cannot add any metadata about any object, since the ontology declares that no object can exist, and there are no attributes nor relationships. Therefore, the framework fails the design goal miserably. It is totally useless. The fi rst thing a developer has to do is to start fi lling in the void. For this purpose, we have created an ontology that is used to extend the empty ontology when the framework is initialized. Our initial ontol- ogy is designed to be useful for mobile personal content, but it can be either totally replaced with other ontologies or, better still, can be extended with new schema objects, properties, and relationships. 21 We occasionally refer to the ontology as a schema since it is schematics of the actual metadata.