An example of the two main types of external representations: (a) a pictorial representation of the occupants of several rooms along a corridor, and (b) a linguistic description of the same information.

rooms in the world. This structural resemblance is often termed analogical. Typically, linguistic descriptions do not have this analogical property because the relationship between a linguistic symbol and that which it represents is arbitrary (de Saussure, 1960). There is no inherent reason why small, furry, household pets should be labelled by the word "cats". If the English language had developed along other lines, cats might well have been designated by the word "sprogdorfs". Even onomatopoeic words (like "miaow") that seem to resemble the sound they represent are really arbitrary, as evidenced by their failure to be used in every language. In Irish, for example, the word for "miaow" is "meamhlach" (pronounced "me-av-loch").

Differences between external representations

The critical difference between written and graphical representations just outlined has several specific implications. Consider another example involving two alternative representations of a book on a desk (see Figure 9.3). There are several ways in which these two representations differ (see Kosslyn, 1980, 1983).

First, the linguistic representation is made up of discrete symbols. The words can be broken down into letters but these are the smallest units that can be used. A quarter of the letter "B" is not a symbol that can be used in the language. However, a pictorial representation has no obvious smallest unit. It can be broken up in arbitrary ways and these parts can still be used as symbols (e.g., the corner of the table, half the spine of the book, or even just a single dot from the picture).

Second, a linguistic representation has explicit symbols to stand for the things it represents (e.g., words for the "book" and the "desk" and the relation between them, "on"). The picture does not have distinct symbols for everything it represents. In particular, there is no explicit symbol for the relation between the

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