When they are solving problems, human beings use both internal repre- sentations, stored in their brains, and external representations, recorded on a paper, on a blackboard, or on some other medium. Some investigators (e.g., Pylyshyn, 1973) have argued that all internal representations are propositional, while others (e.g., Anderson, 1978) have argued that there is no operational way in which an internal propositional representation can be distinguished from a diagrammatic one. Although our discussion may be relevant to this current controversy about the distinguishability of different internal representations, our analysis explicitly concerns external represen- tations.We consider external problem representations of two kinds, both of which use a set of symbolic expressions to define the problem.1. In a sentential representation, the expressions form a sequence corre- sponding, on a one-to-one basis, to the sentences in a natural-language description of the problem. Each expression is a direct translation into a simple formal language of the corresponding natural language sen- tence.2. In a diagrammatic representation, the expressions correspond, on a one-to-one basis, to the components of a diagram describing the prob- lem. Each expression contains the information that is stored at one par- ticular locus in the diagram, including information about relations with the adjacent loci.