"I haven't read my La Rochefoucauld for nothing."

François VI, Duc de La Rochefoucauld, Prince de Marcillac (1613–1680), French nobleman and author of memoirs and maxims. His maxims total about 700, most consisting of just 2 or 3 lines each. The view of human conduct they describe is often summed up by the words "everything is reducible to the motive of self-interest." 

Elliott may have in mind Maxim 8: Les passions sont les seuls orateurs qui persuadent toujours. Elles sont comme un art de la nature dont les règles sont infaillibles; et l'homme le plus simple qui a de la passion persuade mieux que le plus éloquent qui n'en a point. (The passions are the only advocates which always persuade. They are a natural art, the rules of which are infallible; and the simplest man with passion will be more persuasive than the most eloquent without.)