If oratory needs only some device instead of knowledge to produce persuasion, I wonder what such “persuasive device” could be. Isn’t it true that professional knowledge proves to be very (if not most) persuasive evidence? A doctor may not be so good at speeches, but his/her professional knowledge can offer concrete evidences to his/her speeches on medicine and make it very convincing/persuasive. In this regard, isn’t professional knowledge the best persuasive device?