The class-inclusion theory of metaphor was proposed by Glucksberg and Keysar (1990), and directed against the view that metaphoric statements of the form X is Y are implicit comparisons, and their interpretation involves a process of feature matching between topic (‘X’) and vehicle (‘Y’). The authors suggest instead that such sentences qualify as explicit class-inclusion assertions, in which the topic is ascribed to a superordinate ad hoc category typified by and named after the vehicle. I will argue that, although superordinate categories comprising both the topic and the vehicle of a metaphoric statement can indeed be constructed in a large number of cases, they fail to provide an adequate theoretical foundation for the interpretation of metaphor. Furthermore, the prototype status attributed to the vehicle obscures the difference between metaphor and certain metonymic patterns in which the name of a prototypical category member stands for the category as a whole.