The mental model theory of reasoning postulates that individuals reason by constructing models of the situations described by the premises, but the number of explicit models and the information in them is kept to a minimum. The initial models of modal conditionals don't only represent the situations, but they also comment these situations, in terms of permissible and impermissible. The present paper reports three experiments which test the prediction of the mental model theory that a modus tollens inference is easier with a rule that contains the auxiliary verb 'must'. Unlike a normal rule, there is no need to flesh out the models explicitly with a rule containing 'must'. Overall, the results corroborate the prediction of the mental model theory.