Referential density is a concept of ficto-narrative theory put forward by Thomas G. Pavel in his 1986 book, Fictional Worlds. The concept refers to the referential relationship of a text to a fictional world, the ontology of which can be established by a possible worlds approach. A large text that refers to a small fictional world is said to have low referential density, whereas a small text referring to a large fictional world has high referential density. The size of the text is measured in abstract terms as amplitude, which in most cases will correspond to its physical length; exceptions to this may arise in cases of embedded discourses, such as metanarratives (or imaging digressions), which refer to the actual world. For this reason, the form and genre of a fictional work provide only an approximate indication of its size; by the same token, it is possible to refer to the size and referential density of part of a fictional work. The size of a fictional world, in turn, is measured in terms of the sum total of properties applicable to the objects and agents inhabiting the fictional world.