["Invisible"] has four meanings:[1] (1) what is absolutely impossible to be seen, such as things which are subject to another sense; for voice, smell and the like are invisible. (2) That which is naturally visible but is not seen; this is precisely what happens to what exists in [the domain of] privation, such as what is manifest in potentiality when there is no light. (3) That which is called "invisible" in this respect, that it is poorly visible; and such a thing is either what is destructive of sight through an excess of brightness or what is dimness in not being active in respect of a sense. Certainly sight takes part in the invisible, not with what cannot naturally be seen. For with regard to these things, because they are not seen, reason is the discriminating factor through the organ of the sense, and certainly not the sense [itself]; rather, [the sense] is able to apprehend what is visible in accordance with privation. For irrational creatures also discriminate by means of their sight that darkness is something incidental, due to the fact that visible things do not affect it; hence they lie low in their lairs. But perhaps the discriminating presentation, and not sight, is the invisible thing in accordance with privation, at any rate if the senses contain a discriminating factor in the things that are being affected, and in not being affected in darkness they are inefficacious. Hence they are considered only according to a disposition. But if they are not efficacious, how could they discriminate?