Knowledge is based on what is, or truths. The only established truths are the forms. The forms represent true, eternal, unchanging, or facts. Knowledge stems from the idea of forms. One who has knowledge must understand the forms. Only a philosopher has this understanding, and therefore only a philosopher has knowledge.
Contrary to knowledge, ignorance is based on what is not, or untruths. Opinion represents all that remains, therefore opinion is both what is and what is not. The opinion represents all truths other than the eternal unchanging forms. Those who love merely sights and sounds cannot obtain knowledge, for they do not recognize the forms in the sights they see, but only the sights themselves. These lovers of sights and sounds instead have opinions.
A man may see beauty in a woman, but this idea is relative. Compared to another woman, she may not seem so beautiful. Over time, her beauty will fail with age. Unlike the woman, the form of beauty itself will always remain beautiful. The man's thought of the beautiful woman remains merely opinion, while the form of beauty itself represents true knowledge.
Socrates goes on to describe the powers. "Powers are a class of the things that are that enable us - or anything else for that matter - to do whatever we are capable of doing (477c)." He explains that knowledge is the power that allows us to know, while opinion is the power that allows us to opine. However, he states that knowledge is an