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

Alhazen's second stage involves the use of the mind's estimative powers and imagination - what others call "common sense"(96) - to ascertain visual form. By sweeping the centric ray over an object, the eye sends to the mind a series of verified or veridical impressions. These impressions are then certified by estimation and assembled by imagination into an image of the object as a whole.(97) In Latin, the sweeping of the visual ray is termed "intuiting" (intueor), the estimation and assembly of the impressions "certification," and this stage of the visual process "intuition" (intuitio) or "intuitive cognition." The resulting "certified" image comprehends all twenty-two "special sensibles" by which things are known to sight.(98) These visual sensibles include not only light and color, which are perceived by pure sensation, but also distance, position, solidity, figure (including contour and three-dimensional form), size, motion, texture, shadow, beauty, and similarity and dissimilarity. The special sensibles of Alhazen are equivalent to the "separated substances" of the scholastics, as Thomas Aquinas recognizes.(99) Both are visual properties known through sense by comparison. In material objects, these separated substances are always mixed; accordingly, the visual sensibles or "intentions" by which they are comprehended are "composed" in a certified image.(100)

Alhazen insists that intuition provides secure knowledge of individuals and species.(101) Certified images, he explains, are fixed in memory in proportion to the frequency that they are ascertained. Since intuition comprehends similarity and dissimilarity as well as the special sensibles, features common to several certified images are fixed most strongly. In this way, memory becomes a storehouse not only for individual likenesses but also for "universal forms" produced by the accretion of certified images. Unlike Aquinas's species, which are abstracted from sense perceptions, Alhazen's universal forms are conglomerates of visual properties usually found in members of a species. As such, they include what Aquinas terms accidents and "separated substances" as well as whatever - if anything - is visible of substantial form. (For example, the universal form of milk is white for Alhazen, not for Aquinas.) Intuition recognizes individuals through the similarity of new certified images to memory images of individuals, and it recognizes species through the similarity of new certified images to memory images of universal forms. Thus, Alhazen's intuitive vision comprehends the whatness of objects in both their individual and their specific identity. For this reason, Bacon calls it "comprehension through knowledge,"(102) and such fourteenth-century philosophers as William of Ockham define "intuitive cognition" as the immediate and full intellectual knowledge of individual things.(103)