1. Artists began to abandon the relatively stable authority of workshop models in favor of specific examples observed in the material world. This desire to imitate concrete references in nature made it necessary to study the direct rendering of visual data through constant practice in drawing.
2. Artists began to study the antique not only with close attention to detail, but with careful adherence to the formal character of the artifacts, or style, whether through following the system of proportion of the ancient models or details of execution.
3. The formal structure of their finished works became considerably more complex. In pursuing the illusion of visual experience of the material world, artists of the early fifteenth century established a convention of imitating the apparent differences in the size of objects in space through a consistent mathematical system. This demanded extensive preparation through drawings.
4. At the same time the established functions of the artworks in the public and private spaces, both secular and sacred, in which they were viewed continued to require the communication of complex intellectual and spiritual meanings.

Just at this time paper became more plentiful and less expensive and enabled artists to achieve these goals in a practical way. This new system of work placed heavy demands on drawing, and the range of functions it required determined the nature of the drawings that have come down to us.
 

by Michael Miller
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