intrinsic nature of our green that it occupies a definite position in a range of colours and in a scale of brightness, and this position is determined by relations of similarity and dissimilarity to the other elements (shades) of the whole system.
These relations which hold between the elements of the system of colours are, obviously, internal relations, for it is customary to call a relation internal if it relates two (or more) terms in such a way that the terms cannot possibly exist without the relation existing between them — in other words, if the relation is necessarily implied by the very nature of the terms. Thus, all relations between numbers are internal: it is in the nature of six and twelve that the one is half of the other, and it would be nonsense to suppose that instances might be found in which twelve would not be twice as much as six. Similarly, it is not an accidental property of green to range between yellow and blue, but it is essential for green to be related to blue and yellow in this particular way, and a colour which were not so related to them could not possibly be called "green", unless we decide to give to this word an entirely new meaning. In this way every quality (for instance, the qualities of sensation; sound, smell, heat etc. as well as colour) is interconnected with all others by internal relations which determine its place in the system of qualities. It is nothing but this circumstance which I mean to indicate by saying that the quality has a certain definite logical structure.
It will help to make matters clearer if I say a few words about "external" relations. The relation holding between the leaf and the desk is "external", because it is in no way essential for the leaf to be lying there, nor does it belong to the nature of the desk to have the leaf lying on it. The surface of the desk might just as well be empty, and the leaf might be somewhere else. If the leaf happens to have the same colour as a blotter lying next to it, the colour similarity between the two objects is an external relation, for the blotting paper might just as well have been dyed with a different colour. You will notice this important difference: the relation of similarity between the two coloured objects is external, but the relation of similarity between the particular colours as such is internal.
It is clear that in speaking of colours or other "qualities" we can refer to them only as external properties of something: we have to define a
