Despite the contemporary revival of interest in topical invention among rhetoricians and informal logicians, the commonplaces (loci communes) of classical rhetoric have received little attention. When considered at all, they are typically dismissed as sterile or mechanistic substitutes for genuine argumentative invention. A fresh examination of the texts of Cicero and Quintilian, however, suggests that these authors believe that the commonplaces have an important heuristic function, and an effort to understand this function is a matter of interest to contemporary students of argumentation.