Full Idea
Rhetoric is a knack, because it lacks rational understanding of its object or what it dispenses (and can't explain the reason anything happens).
Gist of Idea
Rhetoric is irrational about its means and its ends
Source
Plato (Gorgias [c.387 BCE], 465a)
Book Reference
Plato: 'Gorgias', ed/tr. Waterfield,Robin [OUP World's Classics 1994], p.32
A Reaction
If there are cunning people who have the wrong sort of intelligence for morality, there must be cunning users of rhetoric who know exactly what they are doing.