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

The intellect becomes like corporeal things by thinking the forms in phantasms of individual substances. Aquinas points out that Aristotle attributes to the intellect two kinds of active thinking with respect to sense: apprehension and judgment.(47) Apprehension abstracts the whatness (quidditas), or "what it is," of individual things from phantasms (S, 1.84.7. res.).(48) Through apprehension, the intellect understands individual things as members of a specific class: for example, when phantasia interprets a perception as the appearance of a woman, intellect apprehends what makes this appearance a woman Because every individual is unique, this apprehension is necessarily vague and general. On deeper consideration, the intellect might further apprehend the whatness of the class to which the individual belongs; for example, it might understand that womanhood is female humanity. Aquinas calls the object of this apprehension a "species," or universal phantasm. Once apprehended, universal phantasms are stored in memory, where they aid in perception and thought. As objects of thought, species are comparable to mathematical forms and nounal meanings (S, 1.85.1, 6). Since individual women exist but "female humanity" does not, species are abstractions, not physical entities. Yet like abstract mathematical forms underlying the magnitudes of existing things, species inhere in substance and cannot be considered without reference to real things. And, like nounal meanings, species are, in Aquinas's view, neither true nor false, only more or less completely understood; for example, all competent speakers understand in a vague and general way that the noun gold means precious yellow metal, but chemists can give a precise and unambiguous definition of gold as a specific element. Since the intellect always apprehends by means of species, it cannot fully comprehend individuated substantial forms by apprehension alone.

Instead, understanding material things in their particularity requires judgment. For Aquinas, judgment is "a certain kind of reflection" through which species are compared with phantasms (S, 1.86.1). Like Aristotle, Aquinas terms this process "composition and division."(49) Unlike apprehension, composition and division are subject to "truth and falsehood . . . just as in the complex utterance which is its expression" (S, 1.85.2. ad 3). "Composition" affirms by bringing together two or more unitary concepts called "indivisibles"; "division" denies by separating indivisibles. Aquinas identifies three types of indivisibles.(50) Two are "whatness" and "species"; the third, "separated substances." Separated substances are accidental properties - such as color, size, texture, and so forth which are known by comparison through sense. For example, redness is a separated substance commonly perceived in apples. It is not possible to define redness except through reference to red things like apples, but just as redness does not figure in the definition of these things, so, too, these things do not define what red is. Following Aristotle, Aquinas characterizes separated substances as "privations" standing "in opposition to corporeal things, whose whatness the intellect receives first and by itself" (S,1.85.8 res.). If species are comparable to mathematical forms, separated substances are comparable to mathematical points. All sensible things, Aquinas explains, have magnitude. Since a point has no magnitude, it is not a sensible thing. But when understood negatively as the smallest possible division between perceived things, a point may be sensed as "a sort of sign of division" between magnitudes.(51) It is the same with color and texture, for black is the privation of white, and smooth the privation of rough. Thus, "separated substances" are sensible forms that are not themselves sensible or intelligible things.