“The protreptic text, the so-called protreptikós, is an invitation to produce one's own knowledge. So one might easily think, at first glance, that protreptic was nothing more than rhetoric, and it is probably correct to speak of a particular sort of rhetoric. For protreptic texts use every possible rhetorical means with the explicit goal of engaging with a discipline in one's own way. And that is exactly the point of significant difference from rhetorical structures exclusively concerned to bring an audience to a particular point of view: in the protreptikós, language serves to talk an audience into persuading itself. … The protreptikós does not argue for a particular persuasion, but directs attention to the ways one can achieve certainty for oneself.”

"The work is not the performance", Companion to Medieval & Renaissance Music. (1997). Oxford University Press. ISBN 0198165404.

Speech at the Cambridge Union (March 1924), quoted in On England, and Other Addresses (1926), pp. 94-95.
1924

Source: Rhetoric as Philosophy (1980), pp. 31-32
Context: In the second part of the Phaedrus Plato attempts to clarify the nature of “true” rhetoric. … it does not arise from a posterior unity which presupposes the duality of ratio and passio, but illuminates and influences the passions through its original, imaginative characters. Thus philosophy is not a posterior synthesis of pathos and logos but the original unity of the two under the power of the original archai. Plato sees true rhetoric as psychology which can fulfill its truly “moving” function only if it masters original images [eide]. Thus the true philosophy is rhetoric, and the true rhetoric is philosophy, a philosophy which does not need an “external” rhetoric to convince, and a rhetoric that does not need an “external” content of verity.

Speech at the Cambridge Union (March 1924), quoted in On England, and Other Addresses (1926), pp. 95-96.
1924

ibid
The Rahotep series, Book 2: Tutankhamun
Context: Rhetoric is a dangerous art. It is the manipulation of the difference, one might say the distance, between truth and image [... ] And in our times, that distance has become the means by which power is exercised [... ] Rhetoric has been a force for persuasion since man began to speak, and to convince his enemy that he was indeed his friend.

Introduction, p. 17
Elements of Rhetoric (1828)

Source: Quotes From Judge Judy Cases, Dress, Stand, and/or Speak Properly

J. Agee, trans. (1989), p. 61
Das Geheimherz der Uhr [The Secret Heart of the Clock] (1987)

Authority and persuasion in philosophy (1985)