If you are ignorant of [what a Sophist really is], you cannot know to whom you are entrusting your soul—whether it is to something good or to something evil.

In this passage (312c), Socrates is exploring precisely why Hippocrates is so eager to study with Protagoras that he would wake him in the early morning. Socrates's caution to Hippocrates lays out the importance of the subject of the dialogue that is about to take place. Education, when it concerns in particular the teaching of virtue, involves the alteration of the soul. Before we have been taught what virtue is, we have no way of determining whether the teacher who claims to be able to teach it to us can indeed do this. To be educated is to submit to instruction in a state of ignorance. If we were not ignorant, after all, we would not need to be educated. But this paradox is no mere logical trick. It concerns the future state of our soul, a matter which—as Socrates points out to Hippocrates—is of the utmost importance.