Knowledge is tentative. We can't know anything for sure. What we can have is good opinions. A good opinion is one that helps explain life in a way that helps me get along in the world. Correspondence to ultimate reality is not important.
Values are relative.
Isocrates agrees with Plato in promoting traditional Greek values, but he does it for different reasons. Isocrates believes those values are useful, but not necessarily the true values. Plato wanted to develop in the people a passion for those values because they are real and eternal and never changing. Therefore, they could give a unity of purpose and meaning to Athenian life - a reason for living and participating. Isocrates realized that his relativistic value system did not have the psychological force to draw people together into the common bond of unity and fraternity that would stabilize society.