 While other economic goods, such as crude oil or wheat are generic, interchangeable commodities (given a specific grade of the product), there is only one example of a famous painting such as the "Mona Lisa", and only one example of Rodin's well-known sculpture "The Thinker".While copies or reproductions can be made of these works of art, and while many inexpensive posters of the Mona Lisa and small factory-made replicas of "The Thinker" are sold, neither full-size copies nor inexpensive reproductions are viewed as substitutes for the real artworks, in the way that a consumer views a pound of Grade A sugar from Cuba as a fully equivalent substitute for a pound of Grade A sugar from United States or Dominican Republic.As there is no equivalent item or substitute for these famous works of art, classical economist Adam Smith held it was impossible to value them.