, seeing is to the sense of sight, hearing to the auditory sense, and so on with the other senses severally; while all are accompanied by a common power, in virtue whereof a person perceives that he sees or hears (for, assuredly, it is not by sight that one sees that he sees; and it is not by taste, or sight, or both together that one discerns, and that sweet things are different from white things, but by a part common to all the organs of sense; for there is one sensory function, and the controlling sensory organ is one, though differing as a faculty of perception in relation to each genus,.g., sound or colour); and since this subsists in association chiefly with the faculty of touch (for this can exist apart from all the other organs of sense, but none of them can exist apart from it a subject of which we have treated in our speculations concerning the soul); it is therefore evident that waking and sleeping are an affection of this.