The three dominant views of metaphor emphasize comparison anomaly or dissimilarity, and a somewhat vaguer notion that combines aspects of the first two, called conceptual interaction. In all three views, a central consideration as to the aptness of the metaphor is the similarity of the objects linked by the metaphor (tenor and vehicle). The exact nature of the quantitative relationship between similarity and aptness depends in part on how similarity is conceived. Psychologists have represented those concepts as points in a semantic space, as bundles of features, or as "nodes" in a network of associations. Each of those representations lends itself to a particular measure of similarity or distance; for example, the domains-interaction view, a modified version of conceptual interaction, requires an elaborate notion of distance. What little evidence psychologists have gathered suggests that intermediate degrees of similarity make for the best metaphors, and that metaphors require more time to understand than literal sentences. (RL)