There is a green leaf lying on my desk. My fingers touch it, my eyes see it, I am aware of its shape, its colour, its approximate weight, and so on. You, who are not present in my room, are not aware of any of these properties, but it is possible for me to communicate them to you by describing the leaf. The description expresses its properties; how is it effected, and are there any limits to it? From what has been said before we might be led to think that there must be two kinds of properties: those which can be described and communicated, and those which cannot; the former constituting the structure of the leaf, the latter its material or content. But that would be a mistake, for in a certain sense a complete description can be given of all the properties of the leaf, and it is not in this simple way that we arrive at the distinction between form and content.
The size of the leaf will be described by giving its various measurements, say, in inches; its shape will be communicated by mentioning its similarity to the shape of some well known object (“heart-shaped” etc.) or by giving a drawing of its contour, which, theoretically speaking, could be replaced by a mathematical equation representing the curve of the contour. Similarly, a description of the colour can be given by certain combinations of words such as “dark yellowish green”, “a little darker than the green dress of a certain Madonna by Raphael”, and so forth; and if this does not seem definite enough, I might state the exact physical circumstances under which light of that particular green colour is produced; or, finally, I might send you a piece of paper with a green spot on it and write underneath: “This colour is exactly like that of the leaf on my desk.” In this way I could go on and answer all the questions you could possibly ask about the leaf, without any exception.
All my answers, all my descriptions of the leaf are propositions by which I can communicate to you the whole of my knowledge about the leaf. This knowledge is knowledge of a certain set of facts, and if our former arguments are true my propositions express these facts by conveying to you their logical structure, and nothing but their logical structure.
Most people will find it difficult to see that this is so; they will be inclined to believe that my descriptions contain information about the “material” as well as about the structure of the facts which they express. Even the statements concerning the shape and the size of the leaf do not seem to be
