Art is the creative reintegration of concretes which results in a work representing the artists sense of life (and is interpreted by the viewer via the viewers sense of life).  This creative reintegration is synergistic, and represents completely new views/thoughts which transcends a collection of models.

A specific model is generally a concrete, though there are certainly some models that can be defined as works of art in themselves.  An example might be some of the work done by a modeler/vender named Stonemason. His work (like Urban Sprawl) is so comprehensive (in a very good way) that I personally won’t purchase it, because too much of the sense of the creative is contained in his models. What is left for the 3D artist when Stonemason’s sense of Urban setting is so dominant in the scene?  If I used one of his excellent environments, I would feel strongly compelled to give him credit simply because most of what I could add would be (in my opinion) filler – not much room for originality.  In such a case he is more of a coworker artist than someone selling generic models as props – he deserves credit even if one pays for his models.

Many folks have done some fine scenes with his environments, but the scenes are really dominated by his creation, and the artwork using his comprehensive environments are so dependent on his work that they end up almost as variations of his modeling work.

A concrete would be something like the model of a chair or a book.  They are very generic, and the value they have in a work of art is dependent on how they are used in that artwork – how they are reintegrated as a whole that elicit thought and emotion from the viewer, and greatly transcend any specific model used in the scene.  The viewer doesn’t really notice a particular model when feeling the sense of a well done scene, the models are integrated into a whole, something unique and, yes, original.

Giving credit, certainly for generic models, is not so simple when it is the artist who often brings them to life!  I do think though, if someone uses a donated free model, credit should be given to the models originator.  If you purchased the model in a commercial transaction, it is the discretion of the artist to give credit.  There is little, if any, ethics involved when the models used are a commercial product.