Journal of the History of Philosophy

1. Background

The Aristotelian tradition distinguishes the familiar five external senses from the less familiar internal senses. Aristotle himself did not in fact use this terminology of 'external' and 'internal,' but the division became common in the work of Arab and Hebrew philosophers, and in the Latin West the distinction is taken for granted.¹ I am going to put to one side the obscurities involved in the various internal senses of phantasia, imagination, memory, etc.; I will instead focus on the external senses. These five external senses—sight, hearing, smell, taste, touch—have their familiar counterparts in the external world: color, sound, odor, flavor, and the various tangible qualities. (The tangible qualities, unfortunately, resist being reduced to a single genus, and are typically listed as hot and cold, wet and dry, etc., where this etc. stands in for the complete list that one might or might not be able to give.² ) These sensibilia (I will simply speak of sensibles) were known as the proper or special objects of the external senses.

In addition to these proper objects, the senses were taken to have five common objects—size, shape, number, motion, rest—and also an unlimited variety of incidental objects, the per accidens sensibles—Coriscus, one's enemy, a horse race, etc. These per accidens sensibles are in a way the most interesting and important sense objects of all. They are interesting, first, inasmuch as one of the hardest and most interesting problems about perception is the problem of seeing as, which seems to be precisely the phenomenon at issue [End Page 27] here. They are important, also, inasmuch as genuine perception, as we now think of it, almost always involves the perception of something sensible per accidens. Our everyday sensory experiences revolve around the experience of seeing objects of certain kinds. We can of course step back and decide that what we are really seeing is colors and shapes. But that is not how we ordinarily see things.

Despite their interest and importance, I want to put aside these per accidens sensibles, and focus instead on the common and proper sensibles, both of which Aristotle described as sensible per se (kath' hauta). In his classic statement on the objects of sensation, at De anima II 6, 418a7-25, Aristotle suggests that the grounds for distinguishing the per se from the per accidens are that the former sensibles are capable in their own right of making an impression on the senses. Color itself affects the external senses, and so does size. Coriscus, however, considered as Coriscus, does no such thing. He affects the senses only inasmuch as he is colored, perfumed, etc.³ To focus on the per se sensibles, then, is to focus on those things in the world that are suited to make impressions on the external senses.

Why are some such sensibles proper, whereas others are common? The well-known rule of thumb here, reflected in how we refer to these sensibles, is that each proper sensible can be perceived by only one of the external senses. The common sensibles are those per se sensibles that fail this test. (Note that, of the common sensibles, only number, movement and rest are truly common to all five of the senses; shape and size are perceptible only by touch and sight.⁴ ) It is not clear, however, that this is the appropriate criterion for distinguishing the proper from the common sensibles. Albert the Great, for instance, argues that a proper sensible must meet three tests: (1) It is sensed by only one sense; (2) this sense is not capable of error with respect to it; and (3) the organ of this sense is naturally suited to be affected by it alone.⁵ The last of these tests is problematic, however, for at least two reasons. First, it is problematic for touch, given that the tangible qualities resist being reduced to a single genus. Second, more seriously...