Institutio Oratoria

Moreover, if the class of legal experts is as a rule drawn from those who, in despair of making successful pleaders, have taken refuge with the law, how easy it must be for an orator to know what those succeed in learning, who by their own confession are incapable of becoming orators! But Marcus Cato was at once a great orator and an expert lawyer, while Scaevola and Servius Sulpicius

And Cicero not merely possessed a sufficient supply of legal knowledge to serve his needs when pleading, but actually began to write on the subject, so that it is clear that an orator has not merely time to learn, but even to teach the law.