Final answer:
The main premise of De Oratore is the importance of effective oratory in the political lives of Roman men.
Step-by-step explanation:
The main premise of De Oratore is the importance of effective oratory in the political lives of Roman men. Quintilian discusses the physical aspects of oratory, such as stance and gesture, emphasizing the impact of the entire body on the delivery of a speech. Additionally, he highlights the significance of rhetorical skills in public life, drawing inspiration from the Greeks' mastery of rhetoric and the Romans' desire to emulate them.