A man who knows himself is someone who is dead certain of what he wants and makes all his choices – about career, mate, income level, spirituality, politics – with a view to acquiring it. A man who knows himself is one who cannot entertain the idea that he might be wrong, or that there may be two or more viable ways of interpreting an issue or solving a problem. A man who knows himself projects that arrogance into his judgement of others, and formulates opinions about them that are almost inescapably ill-founded and unshakable. In the life and ideas of a man who knows himself (or even thinks he knows himself), there is no room for the whimsy, ambivalence, doubt, nuance, curiosity and inquisitiveness upon which all creative thought is based.

I agree that "just be yourself" isn't the most helpful advice, but this isn't what it means – the idea is that you should do what you want to do rather than acting like you think people want you to. That's not the same thing as refusing to think or consider that you might be wrong.