Signs, or representations, are of three kinds: Icons, Indices, and Tokens. [—]

The icon represents its object by virtue of resembling it. It thus depends on a simple feeling. Mental association has nothing to do with it. The icon has no generality, because it does not analyze the character it exhibits. There is thus no more generality in the icon than there is in the object. Nor has the icon anything to do with the sense of contact with the world, nor with the actual existence of its object. It is a mere dream. Icons comprehend all pictures, imitations, diagrams, and examples. Every algebraical formula, in so far as it shows the letters connected by signs analogous to the relations between the quantities those letters denote, is an icon: q^q. No quality or character of any kind can be conveyed or made known, except by the means of an icon. With reference to qualities of feeling, this is evident. But it is equally true with regard to relations.