Power of Persuasion

After reading On Rhetoric, I was confused about the specific role the rhetor played in persuading the audience.  Actually, in one passage, Aristotle says he plays no part in it: 

The rhetoric, therefore, does not belong to a single defined genus of subject but is like dialectic and that it is useful is clear--and that its function is not to persuade but to see the available means of persuasion in each case, as is true also in all the other arts. (35)

Aristotle is saying here, basically, that the rhetor's role is merely to elucidate or "make clear" the truths that are already present within the case. For instance, a judiciary committee is presented with a case about a man who cheated on his wife. The rhetor's job is to present the testimonial of the husband's mistress in which she admits to having an affair with him. The rhetor doesn't tell the jury to vote a certain way; they simply present the facts. 

Aristotle describes the qualification for the rhetor in the following paragraph:

for it belongs to the same capacity both to see the true and [to see] what resembles the true, and at the same time humans have  a natural disposition for the true and to a large extent hit on the truth; thus an ability to aim at commonly held opinions is a characteristic of one who also has a similar ability to regard the truth (33)

Basically, it seems to me, that Aristotle is saying that the rhetor's job is to uncover ideas that the audience has already had, but how do you know what's a common belief, or if the audience has really had such a thought before?

The OED defines persuasion as, "the addressing of argument or appeals to a person in order to induce cooperation, submission, or agreement."  Trying to convince someone to agree with a particular position is the act of persuasion. By saying the rhetor gives the audience the means to persuade, is Aristotle saying that the audience must convince themselves to take a particular opinion? In that case, you would be acting on yourself . This is possible. A common phrase is, "I have to persuade myself."

But let's say someone wanted me to go to a party, but I wasn't sure. They told me I would have fun, and I don't get out enough. I think, "I don't get out enough, so I should go." Did that idea really belong to me? Aristotle is saying that even though the rhetor brings our attention to these ideas, they were things we already knew to be true. What if, up until that point, I'd never thought of myself as not going out enough, but because that person made the statement, I changed my mind and decided I don't get out enough. Was that really my idea? If I change my mind and accepted his truth as my truth, was I really the one who had the power of persuasion?

I think Aristotle was trying hard to argue that the power of persuasion belongs to the audience. For instance he says this on emotions: "There are morally valid emotions in every situation, and it is part of the orator's duty to clarify these in the minds of the audience." He's not telling them how to feel (sympathetic, horrified, etc.); he's simply reminding them that as normal human beings, they're supposed to be feeling horrified in certain situations already. Instead of giving the audience more agency over their persuasion, I think it is actually taking it away. The rhetor is supposed to be presenting the truth that is commonly accepted by everyone, but what if the audience accepts what a rhetor is saying because they're aware of that supposition? The rhetor isn't saying something the audience believes to be true, but because it's assumed that the rhetor is reminding them of their own truths (truths that are possibly latent), they have to believe it. If they don't believe it, then they cannot by typified as a common human being; they're abnormal. 

I hope that made sense.