Final answer:
Socrates called knowledge the only true virtue because it is necessary to understand the essence of virtues and choose what is truly good.
Step-by-step explanation:
Socrates called knowledge the only true virtue. He believed that to live a virtuous life, one must have knowledge of the good and understand what is truly good. Socrates believed that ignorance leads to wrongdoing, so it is important to seek knowledge and understand the essence of virtues.