Dialectic was the method of persuasion that Plato used.
Dialectic was the foundation of Aristotle's logos (logic). Plato
believed that the pure factual truth was the most effective and
ethical way to persuade, which is what dialectic was. Dialectic
used no distractions such as ethos (creditability) and pathos
(emotion). Rhetoric is a counterpart of dialectic because
Aristotle's logos is an off-shoot of dialectic.