Sculpture is a solid, three−dimensional form of art which is essentially identified with the material of the work. The accepted fact that the sculpture is three−dimensional does not equate to recognition of the sculpture as solid. There is a peculiar mechanism in recognizing the third dimension, which corresponds to space concept acquired on a two−dimensional plane of picture. This shows that the concept of a solid is more than the mere perception of form, and in a sense it is subjective recognition of this point. However, the concept of solidness requires the effect created by a line projected on the two−dimensional plane, which makes the technical verification of form possible. But the sense of line in a sculpture directly leads the manipulation to the material, or existent substance. This process gives the sculpture its own nature. In other words, the concept of a solid in sculpture is closely related to other factors, such as the production process and the nature of the material, which differ from the mere outer form.