On Alberti's "sign": vision and composition in quattrocento painting - Leon Battista Alberti's commentary 'On Painting'

These initial definitions seem so clear and self-evident that the sentences equating the visible with signs (which they purport to explain) have for the most part escaped critical attention.(64) But I have long been troubled by them. Presented as a consensus view, Alberti's statement that the painter is concerned only with "what is seen by light" marks a fundamental departure from previous opinions. Just three decades earlier, for example, the Tuscan painter Cennino Cennini described painting as "an art for which it is necessary to have fantasia and skill of hand in order to find things not seen, seeking them under the shadow of natural objects . . . thereby showing that which is not as if it were" (emphasis added).(65) Alberti's treatment of the sign, point, line, and surface is hardly less puzzling. Why would he give simple definitions of these Euclidian elements but not define such complex, specialized, and important concepts as historia, ingegno, invention, and function (officio)? Why does he equate the visible with signs, and why define points and lines as certain kinds of signs? Signs for what?

The answers to these questions emerge from a consideration of medieval literature on optics - or perspectiva, as it is called in Latin. Scholars have long acknowledged the importance of this literature for Alberti's discussion of vision. However, most treatments have centered on how optics contributed to his explanation of constructed perspective. Little attention has been given to the broader epistemological issues raised by Alberti's adoption of optics as a model for the cognitive processes involved in painting.(66)

But as Graziella Federici-Vescovini, A. I. Sabra, David Summers, and Katherine H. Tachau emphasize, medieval and Early Renaissance optical science presents vision not simply as a form of perception but as a paradigm for cognitive knowledge of the material world.(67) Because vision is a cognitive process involving inner sense and intellect, optics links sight with semantics, semiotics, and theories of the soul. It makes use of such fundamental Aristotelian concepts as form, substance, accident, quality, individual, universal, species, and whatness. More important, it explains how the intellect might comprehend material things fully as individual objects, instead of merely approximating them to abstract species. For this reason, it provides late medieval and Early Renaissance thinkers, especially those with nominalist leanings, with an alternative to scholastic models of cognition.

In optics, the contribution of Arab thinkers, especially Abu Ali al-Hasan ibn-al-Haytham (ca. 965-ca. 1039), known as Alhazen, is crucial. From the thirteenth century on, Latin translations of Alhazen's major optical treatise circulated under the title Perspectiva or De aspectibus. This treatise influenced every major European writer on optics from Roger Bacon (ca. 1214-1294),John Pecham (d. 1292), and Witelo (13th century) to Johannes Kepler (1561-1630).(68) Moreover, an Italian translation by Guerruccio di Cione Federighi circulated in quattrocento Florence, where it was read by Ghiberti and other artists.(69) Indeed, all the major points of Alhazen's theory discussed here are summarized in book 3 of Ghiberti's Commentarii.(70)