The ability to do this without changing the sense of the metaphor suggests that two different mappings are relevant. Though in most cases metaphors create new concepts only temporarily, for the duration of a sentence, occasionally new entities such as family trees are created that take on an existence of their own. Metaphors are conceptual mappings independent from the perceptual mappings used to express them. In the everyday world, the relation between appearance and functional attributes is often very close because objects' physical structures are lawfully related to their perceptual appearance. For computer entities, in contrast, the relation between their appearance and underlying functionality is almost entirely arbitrary and must be designed.
Metaphors can fuse form and function.
Computer entities have functional attributes but no perceptual ones. By designing a display's perceptual attributes to be similar to those of an everyday analog, the display can simultaneously suggest a conceptual mapping between the functional attributes of the software and the everyday analog, and at the same time create the relation between the display's perceptual attributes and the software's functional ones. The result is that the metaphor acts to fuse the represen- tation and software so that the graphics gains the functional attributes of the software, and the software the appearance attributes of the graphics. The two become one, producing a new sort of graphical entity that, unlike pictures, has functional attributes as well as perceptual ones, affordances as well as appearance. Moreover, if this process of fusing graphics and software is successful, the role of the metaphor is complete.