 From Aristotle’s Topics, logicians of late antiquity had elaborated a system of topical argument, which had been considerably influenced by the needs of Roman lawyers. The focus of topical theory is on discovering arguments, and these arguments are not usually formally valid, but merely plausible. The topical differentiae are the classifications of types of such arguments; knowing the differentiae gives the arguer a ready means to hit upon a persuasive line of reasoning. Suppose, for example, I want to argue that we should praise Cicero. I start trying to think what information I have which might help me to argue this point, and I remember that everyone is full of praises for another orator, Demosthenes. Then I turn over in my mind the list of topical differentiae and I see that the differentia ‘from equals’ will provide me with the argument I need: