Aristotle wonders whether the perception of things that are objects of perception is apprehensible, and [whether] the organs of senses through which we have perception also are objects of perception, [and] why perception does not apprehend each one of them when nothing external is present. For instance, why the eye does not see itself if it is composed out of those things that are seeable (I mean the elements, of course). [...] So what [is] the solution? That perception is in potentiality what precisely it is, but all that is in potentiality is led toward actuality by the action of something that is in actuality. So perception, which is in potentiality, is also led toward actuality by what is an object of perception, since the external object of perception [...] also is in actuality. [...] But if it is led toward actuality by what is an object of perception, and the organ of sense in which perception is also is an object of perception, why it is not led toward actuality by such [an organ of perception]? Otherwise, if it is not possible for perception to apprehend its own organ of sense, which is an object of perception, why does it sometimes apprehend its excesses? For the sense of touch does not apprehend the natural heat existing in any body. But when the heat [present] in us falls into disproportion, such as occurs in the case of fevers, then, [the sense of touch] perceives heat. And similarly, sight does not apprehend the eye which is in its natural condition, such as happens in the case of wet things or the color of tunics. But when it suffers something contrary to nature, such as occurs to those suffering jaundice, it has a perception of the yellowness of the eye; that is why people think that everything is yellow. Similarly also if some other humor falls on the crystalline [lens], it also apprehends the color and its form. Hence also in the beginnings of cataracts, before the activity of the eye-pupil is completely hindered by being blocked, people seem to see mosquitoes and things of that sort.