According to Goodman (1972), two things are similar only if they possess relevant common properties. The relevance of properties, however, can vary with the context and with the goals of the person making the comparison. As a result, similarity is a highly unstable relation, and therefore difficult to use as a base from which to explain other processes, such as analogy, induction, categorization, and metaphor. A recent attempt by Gentner and her colleagues to explain the operations of the comparison process in analogy may run across some of these difficulties. In particular, her structure-mapping theory (Gentner, 1983, 1989; Gentner & Markman, 1997) does not explain how the relevant features are accessed during the early stages of the comparison process. Moreover, progress on this problem will not be made until we have a solution to a more general problem-the problem of explaining how people manage to bring relevant information to bear in the processing of new information. (Dan Chiappe)