Liu and Chou theorize that the rules of Modus Ponens and Modus Tollens have equivalent disjunctive forms. They present an experiment using these forms and conclude that, if applied to disjunctive inferences, the rule of Modus Tollens is not more difficult than the rule of Modus Ponens. Their explanation is based on a successive-conditionalization framework, but, in this paper, it is argued that their results can be interpreted according to, at least, another two theories, the mental models theory and the formal rules theory, and that, therefore, their experiment does not necessarily prove their theses.