Our knowledge of Plato comes through his writings but it is fair to assume that these reflect the method of teaching at his famous Academy, the first school for philosophy in Athens.
Most of Plato's "Dialogues" use the Socratic method, sometimes called the Method of Dialectic, sometimes the Elenchus. A question is put -usually by Socrates. What is piety? What is justice? The answer offered by the respondent is generally an attempted definition which is then refuted. Socrates proceeds to refute successive attempts to define the concept in an attempt to achieve a satisfactory definition and hence true understanding of a universal truth.
Plato conceived a hierarchy of thinking and the level of understanding each sort of thinking could achieve. Opinion could only assist in understanding of the visible world. True knowledge of the universal forms could only be achieved through the higher reasoning process of the dialectic.