…there are three kinds of signs. The first kind consists of Icons, which like all signs are such only by virtue of being interpreted as such, but whose significant character which causes them to be so interpreted is their possessing a quality, in consequence of which they may be taken as representative of anything that may happen to exists that has that quality. Of course there are no signs that are exclusively iconic. But a geometrical diagram, for example, represents, say, a triangle, simply because it is like it.