Plato is concerned with the difference between mere belief and true knowledge because he believes that the truth can often be persuaded, and sometimes cause an injustice. He believes that rhetoric can be used to persuade people to believe something, which may or may not be completely true, by using words. This is similar to how Gorgias thought that rhetoric was like magic and that he was able to persuade people by using it. This lead Plato to believe that rhetoric was like flattery because it was good in the short-term, but not in the long-term. He believed that rhetoric was not a true art and did not come from a true knowledge or skillset.