Plato's refutation of subjectivism.

I was reading Plato's critique of Sophist's subjectivism.Sophists of ancient Greece used to argue that only an individual knows what is good for himself and what is harmful.The only scale of judgement in this regard is subjective conception of every particular individual.But Plato argues that if every individual knows what is good for himself then it follows that no one will make mistakes.But we see that people do make mistakes.So,one cant always say that one knows what is good for oneself.