An experiment is reported which uses a rephrasing task to investigate factors affecting the formation of initial mental models. It was found that both the syntax and the thematic content of the rule affect the initial model set^ formed: the syntax determines the form of the initial model set and the semantics add to this initial set through the representation of subjects' prior knowledge about the situation in question. Specifically, causal content invokes general knowledge about causal relationships which leads to the addition of models representing counterfactual situations in the initial model set. In comparison, familiar content invokes specific knowledge which leads to the completion of existing models in the initial set. Thus, our experiment enables an extension of mental models to be made that accounts for the diffoential effects of general and specific prior knowledge.