Aristotle obviously does take up Plato’s challenge that the rhetor must know his audience and what is likely to persuade them and must know each “class of speech” well enough to choose which is the most advantageous in the situation (Phaedrus 167). I imagine Aristotle didn’t even wince when he read Plato’s criteria. Aristotle probably figured he could get a mental handle on anything, as long as he thought through and analyzed enough examples. And he certainly did that in the Rhetoric. But he doesn’t try to fulfill Plato’s requirement that the rhetor must know “the truth” about his topic (Phaedrus 167). In fact, for Aristotle, “the truth” doesn’t seem to enter into it. Either he assumes that his audience, since they are the educated class, already have “truth” (or at least understanding) within their grasp, or he’s working from the standpoint that the only thing needful is figuring out exactly how best to persuade people (the uneducated). Aristotle is looking for truth in this treatise, but it’s the truth of “how best to persuade people,” not the truth of “what to persuade people of.” (He even recommends rhetors use a logical fallacy when the situation requires it (235).)
So where Plato assumes the rhetor must know the Truth, must be a philosopher, Aristotle allows for a practical art of rhetoric in which the rhetor must have the knowledge to enable him to persuade his audience in whatever situation he finds himself. I think elsewhere Aristotle makes the distinction between “theoretical,” “practical” and “productive” knowledge, while Plato wasn’t willing to allow for any knowledge being absolute knowledge.
It also seemed that Aristotle answered Plato’s call to know the “soul” of one’s hearers in way different from what Plato imagine. Plato seemed to be asking for a much more absolute knowledge of the “soul” of the hearer. In the context of the Phaedrus, it sounds as if Plato means “soul” in the metaphysical sense, while Aristotle tackles the problem from a very merely psychological point of view. Plato wants the rhetor/philosopher to know the metaphysical nature of the “soul,” while Aristotle wants the rhetor simply to know what can be learned about the soul from observation and analysis. One deductive, the other inductive.
Overall, though, I can see why Aristotle’s Rhetoric has been immensely influential. I don’t know if he invented the enthymeme (or merely developed it), but it’s a masterful way to transfer the logical power of dialectic to the needs practical rhetorical situations. Before Aristotle, there was a stalemate between philosophy and sophism. One was intellectually respectable but not practical. The other was practical but not intellectually (or morally) respectable. Aristotle made rhetoric both intellectually respectable and immensely practical.