Art models are often paid professionals who pose or provide the human figure in a work of art. Though professional, art models are usually anonymous and unacknowledged subjects of the work. Models are most frequently employed for art classes or by informal groups of experienced artists that gather to share the expense of a model. Models are also employed privately by professional artists. Although commercial motives dominate over aesthetics in illustration, its artwork commonly employs models. For example, Norman Rockwell used his friends and neighbors as models for both his commercial and fine art work. An individual who is having their own portrait painted or sculpted is usually called a "sitter" rather than a model, since they are paying to have the work done rather than being paid to pose.
Mimicry occurs when a group of organisms, the mimics, evolve to share perceived characteristics with another group, the models. The evolution is driven by the selective action of a signal-receiver or dupe. Birds, for example, use sight to identify palatable insects (the mimics), whilst avoiding the noxious models.
The model is usually another species, except in cases of automimicry. The deceived signal-receiver is typically another organism, such as the common predator of two species. As an interaction, mimicry is in most cases advantageous to the mimic and harmful to the receiver, but may increase, reduce or have no effect on the fitness of the model depending on the situation. The model may be hard to identify: for example, eye spots may not resemble any specific organism's eyes, and camouflage often cannot be attributed to a particular model.
In library and information sciencedocuments (such as books, articles and pictures) are classified and searched by subject - as well as by other attributes such as author, genre and document type. This makes "subject" a fundamental term in this field. Library and information specialists assign subject labels to documents to make them findable. There are many ways to do this and in general there is not always consensus about which subject should be assigned to a given document. To optimize subject indexing and searching, we need to have a deeper understanding of what a subject is. The question: "what is to be understood by the statement 'document A belongs to subject category X'?" has been debated in the field for more than 100 years (cf., below).
Hjørland (1992, p.185) defined subjects as the epistemological potentials of documents.
This definition is in line with the request oriented understanding of indexing quoted below. The idea is that a document is assigned a subject to ease retrieval and findability. And the criteria for what should be found - what constitutes knowledge - is in the end an epistemological question.