may be defined as the faculty of discovering the possible means of persuasion in
reference to any subject whatever. This is the function of no other of the arts,
each of which is able to instruct and persuade in its own special subject; thus,
medicine deals with health and sickness, geometry with the properties of
magnitudes, arithmetic with number, and similarly with all the other arts and
sciences. But Rhetoric, so to say, appears to be able to discover the means of
persuasion in reference to any given subject. That is why we say that as an art
its rules are not applied to any particular definite class of things.