de Anima 3.1 (425a14-425a29) (the Common Sense)

Further, there cannot be a special sense-organ for the common sensibles either, i.e. the objects which we perceive incidentally through this or that special sense, e.g. movement, rest, figure, magnitude, number, unity; for all these we perceive by movement, e.g. magnitude by movement, and therefore also figure (for figure is a species of magnitude), what is at rest by the absence of movement: number is perceived by the negation of continuity, and by the special sensibles; for each sense perceives one class of sensible objects. So that it is clearly impossible that there should be a special sense for any one of the common sensibles, e.g. movement; for, if that were so our perception of it would be exactly parallel to our present perception of what is sweet by vision. That is so because we have a sense for each of the two qualities, in virtue of which when they happen to meet in one sensible object we are aware of both contemporaneously. If it were not like this our perception of the common qualities would always be incidental, i.e. as is the perception of Cleon's son, where we perceive him not as Cleon's son but as white, and the white thing happens to be Cleon's son.

The Common Sense

• The five senses
• There are senses, sense-organs, and objects of sense perception
• sight, eyes, visible things
• smell, nose, smell-able things
• etc.
  
• Each sense and sense-organ has a proper object. A proper object is an object which that sense alone can sense.
  
• Sight can sense visible things
• Hearing can sense audible things
• Taste can sense gustatory things
• Smell can sense olfactory things
• Touch can sense tangible things
• The common sense and its objects
  
• What about "common" things such as motion, shape, rest, motion, magnitude, number, and unity?
  
• Take shape:
  
• Sight can sense shape. Thus shape is an object of sight.
• But touch can also sense shape. Thus shape is an object of touch.
• Thus it is not a proper object of sight or touch. Another way to say that is that shape is not sensed by either sight or touch exclusively.
• The same is true of the other common things as well (rest is not sensed by sight alone, nor is magnitude, unity, etc.)
• Thus Aristotle calls them common sense objects: they are objects held in common by more than one sense. He also talks of the common sense (a little more on which in a sec)
  
• The particular senses that do perceive these common sense objects perceive these objects "coincidentally."
• What Aristotle means by coincidence here seems to be that a common object is only coincidentally sensible:
  
• it follows from the proper objects of a sense that it can perceive a common object (shape is visible, so sight can see it: shape is tangible, so touch can feel it), but it is not part of the essence of that sense to sense the common objects (the essence of sight is that it senses visible objects: shape just happens to be visible: it is coincidental to shape that it is visible).
• So these common objects are like the proper sense objects in that we need senses to perceive them, but they are not limited to one particular sense.
  
• What about motion in particular? Aristotle's sentences at 425a16ff are confusing.
  
• How do we perceive magnitude by motion? (425a17)
• The idea is either that we have to physically move around to get a real idea of an object's magnitude, or that we have to do mental movement (calculation, guessing, etc.) to form an idea of an object's magnitude.
• What about this idea of perceiving the object of one sense by another sense: the sweet by sight, for instance?
• it is not problematic to say that when we see sugar, we see white (assuming it is white sugar, as most sugar nowadays is).
• it is also not problematic to say that when we taste sugar, we taste sweet.
• and it is not problematic to say that we have a way of recognizing that the same substance is the white we see as well as the sweet we taste.
• Aristotle may thus be saying that when we taste the sweet white thing, although the proper object of the sense of taste is sweetness, it is also incidentally a white thing, and so we might say that we are tasting a white thing.
• He may also be saying that once we have a concept of sugar, when we see white, we recognize that it is also sweet (even without tasting it): we see the sweet thing.
• Aristotle wants to make it clear that the common objects of perception mentioned above are not proper objects of any special sense or sense-organ. If they were, we would see a white shape, for example, in the same way that we see sugar as a white sweet. Thus we need a common sense to perceive the common objects.
  
• What about the Cleon example?
• This is presumably meant to illuminate the idea of the objects of the common sense: thus our interpretation of the Cleon example ought to offer some illumination of that idea. To the extent that it does not, we ought not to think it is the right interpretation.
• When we see Cleon (or anyone we know), we do not actually see Cleon. We see color, brightness, contrast in some pattern on our visual field. Nonetheless, we say that we see Cleon. Aristotle wants to say that that is the same way that we see an object of the common sense such as a white sphere: we actually proper-to-the-sense-of-sight see a whiteness that has a certain pattern on our visual field, and the movement of our mind identifies that as a white sphere. Thus we say that we see a white sphere. We could also perceive the sphere by touch, in which case we might not know it is white, but we would call it a sphere.
  
• So too in the case of Cleon, we hear a certain audible bunch of sense data, or we see a certain bunch of visual sense data, or smell a certain bunch of olfactory data, and as a result we say that we hear Cleon, see Cleon, or smell Cleon. We do those things in the same way that we see a sphere or feel by touch a sphere.
• Aristotle wants to claim that there is a way in which we see the common sense objects that is not coincidental to the special senses (that is, the 5 senses). He postulates a common sense for that.
  
• He also repeatedly says things like that we perceive Cleon or that we see Cleon: it would be nice if his theory of perception included a robust way for us to see and perceive such things and not just the proper sense objects and the common sense objects. Otherwise he will be constantly talking loosely: that would not be so good.