 Pictures are distinguished from symbols of other sorts in virtue of the distinguishing characteristics of pictorial symbol systems. In particular, pictorial symbol systems are syntactically and semantically dense. That is, given any two marks, no matter how small the difference between them, they could be instantiating two different characters, and given any two characters, no matter how small the difference between them, they may have different referents (Goodman 1976, 226–227; Goodman, Elgin 1988, Chap. 7). Hence, pictures are grouped together with such things as diagrams, ungraduated instruments of measurement, and maps—with those symbols, that is, for which, in simpler words, any difference may make a difference: any difference in a mark may correspond to a different character, and any difference in the character may stand for a different correlation to the field of reference. Even a simple picture, for Goodman, is dense, in the sense that any, however small, mark on the canvas may turn out being relevant to pictorial meaning. Whatever the merits of, or problems with, Goodman’s technical analysis, the notion of density is certainly one way to account for what other thinkers—most notably Kendall Walton (1990)—have referred to as an “openendedness” in the investigation of pictures.