Some of the major differences between two external representations of the same situation.

book and the desk. "On-ness" is shown implicitly by the way the book and the desk are placed; that is, "on" cannot be represented by itself but only in a given context.

Third, in the linguistic representation the symbols are organised according to a set of rules (i.e., a grammar). One cannot say "on is table the book" and have a meaningful combination. These rules of combination exploit the fact that there are different classes of symbols (e.g., nouns and verbs). Pictures do not seem to have grammars of the same sort in that (i) they have less distinct classes of symbol, (ii) if there are rules of combination they are less constrained than those in a linguistic representation.

Fourth, the linguistic representation is abstract in that the information it characterises could have been acquired from any form of perception (e.g., by touch, by vision) and bears no direct relationship to a given modality. In contrast, the picture is more concrete in the sense that, while the information it represents could have been acquired from a variety of perceptual sources, it is strongly associated with the visual modality.

Differences between internal, mental representations

Many of the points we have made about external representations have parallels in our internal, mental representations (see Table 9.1). First, mental representations only represent some aspects of the environment (whether that environment be the external world or our own imagined world). Second, the difference between written and graphical representations is paralleled in mental representations, by the difference between propositional and analogical representations. Propositional representations are language-like representations that capture the ideational content of the mind, irrespective of the original modality in which that information was encountered. Analogical representations tend to be images that may be, for example, visual, auditory, or kinetic.

Propositional and analogical representations also reflect the detailed differences between types of external representations. Propositional representations are discrete, explicit, are combined according to rules, and are abstract. They are abstract in the sense that they can represent information from any modality; but it should also be stressed that, unlike the words of a language, they usually refer to distinct and unambiguous entities. That is, the propositions for the example in Figure 9.3 [represented as on(book, desk)

to distinguish it from the linguistic representation] refer to a specific book and a specific desk and to a specific relationship

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