purely formal in the sense explained, for spatial structure, although "space" may justly be considered as a "form" of natural things or events, is no itself logical structure, for how could it be "spatial" if not by virtue of its content? If the shape of the leaf were described by the equation of its boundary curve it would probably be admitted that the mere equation as such contained nothing intrinsically spatial and could, therefore, impart nothing but the logical properties of the curve. But on the other hand the equation itself does not communicate anything anyway; it pictures the outline of the leaf only in connection with, and by means of, an explanation of the terms composing the formula, the terms must be interpreted as signifying spatial quantities (coordinates), and it is in this way that the content "space" seems to be brought into description: indirectly but no less essentially than appears to be done by actually repeating the contour in a pencil drawing. — Under these circumstances it seems difficult to understand and prove our assertion that only structure can be communicated and that content is inexpressible; it does not appear to be true even for the spatial form of our leaf — how could it be true for its colour!
Later on we shall have occasion to speak of spatial form — so we may put off the consideration of this point and confine ourselves to the analysis of expressions which deal with "quality", that is in our case: with the greenness of the leaf. How do those expressions communicate the colour, and in what sense is it true that they communicate nothing but its structure ? What can we mean by the "structure" of a quality?
7. Structure and internal relations.
Let us first examine the verbal expressions of our ordinary language, i.e. the sentences and their words by which I give a description of our particular green colour. We easily discover an essential feature which they all have in common: they assign to the "green" a certain place within a comprehensive system of shades, they speak of it as belonging to a certain order of colours. They assert, for instance, that it is a bright green, or a rich green, or a bluish green, that it is similar to this, less similar to that, equally dark as that, and so on; in other words, they try to describe the green by comparing it to other colours. Evidently it belongs to the
