 By definition, the living things that we call animals are those that have the power of perception. Aquinas accepts the conventional list of the five senses—sight, hearing, smell, taste, touch—and argues that we arrive at this list of five because there are five discrete kinds of qualities that make an impression on our bodies. These objects are the proper sensibles—color, sound, odor, flavor, hot–cold—so-called because they “make an impression on the senses primarily and per se” (ST 1a 78.3 ad 2). In contrast to these are the common sensibles—size, shape, number, motion, rest—which “do not move the senses primary and of themselves, but on account of a sensible quality, as a surface does on account of its color” (ibid.). Whereas each proper sensible is capable of being perceived only by its corresponding sense, the common sensibles are so-called because they can be grasped by multiple senses, as when we both see and hear a thing move. To these two categories of sense objects Aquinas adds a third category, the accidental sensibles, as when I see that someone is alive (Comm. de anima II.13.184–90). This is, however, not a purely sensory operation, but an act of seeing that occurs in conjunction with a conceptual judgment that the observed motions are a sign of life. (For discussions of the perceptual process see Pasnau 2002 chs. 6, 9; Lisska 2016; De Haan 2019; Cory 2022.)