Since the mental picture is variable, the judgement we form of it must be variable also. There are many reasons for this.
[171] In the first place there are the innumerable differences in living creatures, differences concerned not with a single aspect, but practically with all; differences in birth, in structure and equipment; differences in food and mode of life; differences in predilections and aversions; differences in their sense-activities and sense-movements; differences in the peculiarities which arise from the innumerable ways in which body and soul are affected.
[172] For leaving out of sight for the moment those who form judgements, consider examples among the objects of such judgements. Take for instance the chameleon and the polypus. The former, we are told, changes its colour and grows like the kinds of soil over which it is its habit to crawl; the latter grows like the rocks to which it clings in the sea, and we may fairly suppose that this power of changing to various colours is given them by protecting nature as a remedy against the danger of capture.
[173] Again, have we not seen the dove’s neck change in the sun’s rays into a thousand different hues, sometimes scarlet and dark blue, or fiery or like red-hot coal, again yellow and then ruddy, and all other kinds of colour, so numerous that it would be difficult to give even their names in full?
[174] Indeed it is said that in the land of the Scythians who are known as the Geloans a most extraordinary animal is actually, though no doubt rarely, found called the elk, in size equal to an ox, but with a face shaped very like a deer. The account given of this creature is that it always changes the colour of its hair into that of the places, trees, or any imaginable thing near which it stands, and owing to this similarity of colour, we are told, it is not observed by passers-by, and this fact rather than its bodily strength makes it difficult to catch.
[175] These and similar phenomena are clear proofs of the impossibility of apprehension.