There are three modes of representation and three corresponding genera of representamens, these being icons, indices, and symbols. [—]
An icon, likeness, or image is a representamen whose representative force depends solely upon characters which it possesses materialiter* and which it might equally possess though its object had no existence.
For example, a geometrical figure of a triangle is an icon. For though no representation can take place without an object and an interpreter, yet it is the character which the shape has, in the sense in which anything really has characters, which makes it an image of any strict mathematical triangle there may be.
It is the appearance which constitutes the image; and any physical existence there may be connected with it is extraneous to it. Considered strictissime, it resides in the consciousness of the moment, and usually determines its interpretant by “association by resemblance,” calling it up out of the depths of memory. But in practice, it is impossible to keep up to such excessive strictness of language. One sign frequently involves all three modes of representation; and if the iconic element is altogether predominant in a sign, it will answer most purposes to call it an icon.
* That is, […] really, not representatively. Man materialiter consists of three letters, but formaliter of body and soul.