 One of the most controversial aspects of Cicero’s actions against Catiline was the execution without trial of five of the alleged co-conspirators, an act that ultimately led to Cicero’s exile. Is mere likelihood sufficient to justify the weight of one’s actions in such cases? Should not certainty be the only proper basis? And is a criterion of plausibility even intelligible without a notion of certainty? Whatever we think of Cicero’s answers to these questions, he leaves us in no doubt about why the questions matter. Even at its most abstract, philosophy’s relation to the concrete and practical lies at the heart of his enterprise.