Quintilian, and J.S. Watson. Institutes of Oratory. Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book 12 Chapter 2.
Based on the concept of an orator as a good man, Quintilian goes on to say that morality is something which can and should be cultivated (Quintilian XII.2.1). A meaningful sense of morality is grounded in understanding moral principles (Quintilian XII.2.3). Quintilian finds these principles in the various philosophers. He does not, however, think orators need to become hilosophers. The two fields are distinct (Quintilian XII.2.7). Nonetheless, the different types of philosophy can inform the orator. Quintilian discusses these in brief, according to their branches: dialictics, ethics, and natural philosophy (Quintilian XII.2.10).
Dialectic, the philosophy of argumentation, is very relevant to an orator. Orators use verbal arguments with precision (Quintilian XII.2.11). Moral philsophy deals with the determination of good and evil. This is closely related to the work of an orator. Very few legal cases do not consider right and wrong (Quintilian XII.2.16). The general application of specific moral principles moves an orator from moral philosophy into oratory which is concerned with policy (Quintilian XII.2.18). Quintilian ties this to natural philosophy, in which the orator needs to understand the overarching themes including prophecies and divine actions (Quintilian XII.2.21).
Quintilian asks about appropriate training. He cites a number of philosophers and rhetoricians, concluding that an orator can draw on various sources (Quintilian XII.2.27). The good orator studies ideas from all times and places to develop the most sound moral foundation (Quintilian XII.2.31). This will develop knowledge and boldness.