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

A central thesis of De anima is that phantasms are necessary for thought. "For the thinking soul," Aristotle explains, "phantasms serve as if they were the contents of perception. . . . That is why the soul never thinks without a phantasm" (431a14-16). Yet phantasms are comprehended differently by sense and intellect. "While the faculty of sensation is dependent upon the body," Aristotle reasons, "thought is separable from it" (429b4-5). Accordingly, while "phantasms are like sensuous contents except in that they contain no matter" (432a9), the objects of intellect are "separated" from their sensuous contents as much as possible (429b21-22). "The faculty of thinking. . ." Aristotle insists, "thinks the forms in the phantasms. . . . Thought is the form of forms, and sense the form of sensible things" (431b2, 432a2). Aristotle cryptically describes the potential of intellect to abstract forms from phantasms as "a virtue for becoming all things" (430a14) and the activity of intellect in thinking these forms as "a virtue for making all things" (430a15). Consequently, he holds that "in every case the mind which is actively thinking is the objects which it thinks" (431b 16-17; cf. 429b20, 431a1).

Aristotle's idea that the thinking mind is the objects thought poses a problem. Since the intellect thinks forms without matter, how can it think bodily things, composed of matter? Saint Augustine solves this problem by proposing that active thinking is an interior vision of things in their true, immaterial reality as divine ideas. Thomas Aquinas, however, rejects the immanent Platonism of Augustine's position. He argues that if knowledge originates in sense, thought has to comprehend not just abstract ideas but also the material qualities of bodily objects.