SENSE OBJECTS IN GENERAL
IN treating of each sense we must first discuss sense-objects. We speak of a sense-object in three ways: two [kinds of sense-objects] are perceptible essentially; one incidentally. Of the two former, one is proper to each sense, the other common to all.§ 383
Now, I call that the proper object of each sense which does not fall within the ambit of another sense, and about which there can be no mistake,--as sight is of colour, and hearing of sound, and taste of savour; while touch has several different objects. Each particular sense can discern these proper objects without deception; thus sight errs not as to colour, nor hearing as to sound; though it might err about what is coloured, or where it is, or about what is giving forth a sound. This, then, is what is meant by the proper objects of particular senses.§§ 384-5
Now the sense-objects in common are movement, rest, number, shape, dimension. Qualities of this kind are proper to no one sense, but are common to all; thus a movement is perceptible both by touch and by sight. These, then, are the essential objects of sensation.§ 386
To be a sense-object 'incidentally' is said, for example, of a white object that is the son of Diares. This is perceived incidentally because whiteness happens to belong to what is perceived: but the sense is unaffected by that object as such. Of objects essentially sense-perceptible, the proper are properly such; and to these the essence of each sense is naturally adapted.§§ 387-98