- Order structures are operational structures possessed by every adult. Adults are adept at ordering objects according to a given dimension, at solving transitivity problems, and the like. To study that structure in the adult, we devised a challenging modification to the classic experimental method. Instead of requesting our subjects to order a set of sticks of different lengths, we described a seriation algorithm, whereas their task was to determine how an initially random array of sticks of different sizes would be arranged after completion of the procedure. The subjects were not allowed to touch the sticks, or to use any other external aid. This prevented them from discovering the solution empirically, by performing the procedure and seeing the result, and forced them to rely entirely on mental means. They had to build some mental representation of the spatial, temporal, and size relations of the sticks, as they are displaced by the algorithm. This enabled us to study how mental simulation is used in problem solving. Important theoretical disagreements are found in the literature on mental imagery. Some see it akin to visually perceived images, whereas others view it as similar to, or even as a special kind of description whose properties are wholly dependent upon the subject’s knowledge (Dennett, 1978; Kosslyn, 1980; Kosslyn and Pomerantz, 1977; Fodor, 1975; Piaget and Inhelder, 1971/1966). As Furth (1975) remarked: “figurative knowledge is only as good as the underlying operative knowledge.” Forcing the subjects to work out the effects of the procedure in their heads seemed a likely way to give more flesh to the theoretical considerations in the literature.