158 Personal Content Experience: Managing Digital Life in the Mobile Age TERM DEFINITI ON Instance object An object created from schema object. It is actual metadata and can be Metadata object, Property, or Relationship. Schema object Defi nition of metadata instance item, i.e., part of ontology and is meta-metadata. Content object Correspond to the real world objects, identifi ed by URIs. Metadata Metadata instance object containing properties, relationships, object and events. Contains metadata for some Content or Virtual object. Virtual object Metadata object that has no corresponding Content object, i.e., it is pure metadata. Property Describes characteristics of an object, i.e., metadata tag or attribute. Event Describes interactions with content objects. Relation Describes dependencies between content objects. Schema Describes all defi nitions in metadata. Ontology A class hierarchy which describes each possible metadata object type. Table 5-1. Defi nitions for the most important terms used in the rest of this chapter. to present the metadata. One application developer may measure the length of the song in seconds, another one in minutes and seconds, or a third may make a small programming error and store the fi le size instead. Interoperability between applications will be quickly lost, since no matter how devotedly we documented the attributes, developers will understand them in a slightly different way or use different attri- butes to describe the same value. One developer may use kilobits per second as a unit for bitrate, whereas another one could prefer bits per second. Surprisingly, even metadata standards have ambiguous defi nitions. For example, EXIF 2.2 defi nes the artist tag as “This tag records 18 the name of the camera owner, photographer, or image creator.” We claim that to solve these problems, we need clearly defi ned ontologies. In philosophy, ontology refers to the study of existence. In essence, it aims at answering the question “What really exists?” In computer 18 http://www.exif.org/Exif2-2.PDF